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22102088744

Med

K5623

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Digitized by the Internet Archive
in 2017 with funding from
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A MANUAL OF
E L E M E N T A R Y ZOO L O G Y

Frontispiece ]

Members of a snow fauna.

An example of the possession, by a group of different animals living in the same
locality of common features, which enable them to support life there (see p.
662). Note the thick, white clothing, which keeps them warm and renders'
them inconspicuous. The animals shown, which are drawn from an exhibit in
the British Museum (Natural History), are the Arctic fox, Stoat or Ermine,
Weasel, Mountain hare, Willow grouse and Ptarmigan. They come from the
south of Norway, and assume this covering in winter. Others, such as the
Polar bear, Snowy owl, and Greenland falcon, living amid perpetual snows, are
clad in the same way all the year round.

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OXFORD MEDICAL PUBLICATIONS

A MANUAL OF

E L E M E N TAR Y
Z O O L O G Y

BY

L. A. BORRADAILE, Sc. D.

FELLOW OF SEIWYN COLLEGE, CAMBRIDGE, AND
SOMETIME LECTURER IN ZOOLOGY IN THE UNIVERSITY

ELEVENTH EDITION

OXFORD UNIVERSITY PRESS

LONDON NEW YrORK TORONTO

1945

-'I.* u>\i KtLK^ITY'

amen house, e.c.4
London Edinburgh Glasgow New York
Toronto Melbourne Capetown Bombay
Calcutta Madras
HUMPHREY MILFORD
publisher to the university

A

a

First Edition ...»
Second „ . . . * |

Third

>* • • • • *

Fourth

♦ «• o o •

Second Impression
Fifth Edition ....
Sixth

' ' • » • • .

Second Impression Revised
Third Impression .

Seventh Edition
Second Impression
Eighth Edition
Ninth

» • *

Second Impression
Tenth Edition .

:'BL. CQMr INSTITUTE*

l LIB^ ARY

[Coll

welMOmec

Cal!

No,

the|

venth Edition

• igig
1918
1920

1923

1924
1926
rg28

1930

1931

1932
193 1
1935

1938

1939
194J
1943
^945

I BOOK
PRODUCTION
I WAR ECONOMY
[ STANDARD

A P E R AND BINDING OF
BOOK CONFORM TO THE
AUTHORIZED ECONOMY STANDARDS

PRINTED IN GREAT BRITAIN BY

MORRISON AND GIBB LTD.,

LONDON AND EDINBURGH

PREFACE TO THE ELEVENTH

EDITION

For this edition the book has undergone a thorough
revision, and parts of it have been rewritten. I have added
in small print a good many “ asides ” which I hope will
be of use. to students who have advanced: some way in the
study of the science, and perhaps occasionally to teachers.

The principal alterations and additions concern the
maturation of the gametes, the functioning of nervous
systems, the relation of excretory processes to the environ-
ment, the history of the Vertebrata, the embryology of
Chordata, and evolution ; but there are many others.

Some fifty of the figures are new.

L. A. BORRADAILE.

Selwyn College, Cambridge,

1944-

CONTENTS

CHAPTER I

PAGE

Introductory : The Animal Organism . . . . i

The Frog :

CHAPTER II

External Features and Body-Wall . 31

The Frog :

CHAPTER III

Viscera and Vascular System ... 57

The Frog :

CHAPTER IV

Nervous System and Sense Organs . . 84

The Frog :

CHAPTER V

Histology, the Germ Cells, Death . . 103

Amoeba

CHAPTER VI

. 14°

CHAPTER VII

Flagellata J51

Monqcystis

CHAPTER VIII

. . . - • • • • ♦ 1 5 /

tx

X

CONTENTS

CHAPTER IX

Paramecium and Vorticella. Protozoa

CHAPTER X

I he Protozoa as Parasites of Man

Sponges .

CHAPTER XI

CHAPTER XII

Hydra. Polyps and Medusa. Ccelenterata

Flatworms

CHAPTER XIII

CPIAPTER XIV

The Earthworm. Annelida,
tecture .

Triploblastic Archi-

CHAPTER XV

The Crayfish. Arthropoda .

CHAPTER XVI

The Cockroach. Insects

CHAPTER XVII

PAGE

161

179

200

208

239

255

2S7

322

The Nematoda. Parasitism .

• 352

CONTENTS

CHAPTER XVIII

l * J A

The Swan Mussel. Mollusca

PAGE

• 369

CHAPTER XIX

The Starfish. Echinoderms

389

CHAPTER XX

The Lancelet. Chordata

4°4

CHAPTER XXI

The Dogfish .

• #

4T9

CPIAPTER XXII

Cold-blooded Vertebrata

460

CHAPTER XXIII

The Pigeon .

CHAPTER XXIV

The Rabbit .....

CHAPTER XXV

Mammalia

CHAPTER XXVI

Reproduction and Sex .

CHAPTER XXVII

« 49°

• 517

. 566

» 592

Embryology

. 605

Xll

CONTENTS

CHAPTER XXVIII

Classification

• i

PAGE

• 675

CHAPTER XXIX

Evolution

• •

. 687

CHAPTER XXX

The Animal in the World .

• •

711

APPENDIX: Practical Work

• • •

729

* • » *

• • • *

763

INDEX

TO THE STUDENT

1. If you are using this book only in preparation for a preli?ninary
medical or some similar examination , you should begin by reading
the first chapter ; then read the large print of those chapters which
deal with the animals specified in the schedule of your examination,
and section A of the Appendix ; then by means of the Index find
and re-read the passages on the various general topics which come
within the scope of the examination. At a later stage in your

medical studies you may find useful those pages which deal with
parasites of man.

2. If you are taking a general course in Zoology , read straight

through the book.

3. In any case (a) read at least twice, once to get a general view of
the subject, and a second time to learn its details ; (b) do not omit
to look up the references given from one page to another ; ( c ) make
full use of the figures, identifying each detail they contain as you
come to it in the text, and afterwards studying each figure by itself
with the aid of the explanation attached to it. Note that some of
the figures needed for certain chapters are placed with the corre-
sponding paragraphs of the Appendix.

4. Note also that discussions of various general topics (such as
digestion, the functions of the blood, the lay-out of the vertebrate
skeleton, the general features of joints and muscles, histology, etc.),
are to be found in the earlier chapters, particularly in those on the
frog, and are not repeated in those which treat of other animals when
mention is made of special features of the latter. Therefore if, for
instance, the dogfish or the rabbit is the animal which is treated in
most detail in your course of study, in reading for it you must look
up these topics in the earlier chapters. A number of references are
given to help you in this.

5. You will find the Index of use not only in searching for particular
pieces of information, but also in getting a connected view of all that
you have read on any subject.

XJJl

A MANUAL OF
ELEMENTARY ZOOLOGY

CHAPTER I

INTRODUCTORY: THE ANIMAL ORGANISM

Biology.

Zoology, the science of animals, is a branch of Biology,
the science of living beings. Of such beings
there are various kinds besides the animals ;
but all the kinds have important features in common.
Rightly to understand any animal is therefore to com-
prehend properties which it shares with non-animal living
beings as well as its purely animal characters. Thus at
the outset of our study of Zoology it is desirable that we
should spend a little time in considering the nature in
general of living beings.

Out of the multitude of material objects that surround us,
we distinguish some, as alive, from the rest,
“ uf8iiess.”nd which are lifeless — that is, are dead or have
never been alive. The question before us is,
first, what this distinction rests upon : What is the property
of living things which causes us to contrast them with other
objects ? On this will follow a second question : WTat
further characteristics are common to all living beings ?

The first question is not difficult to answer. We say
that objects are alive when we observe that
they take action -that they “ do things ”■ —
and further that the things which each of them does are
such as should be beneficial to it and to its own kind. A

Life

I

2

MANUAL OF ELEMENTARY ZOOLOGY

lifeless machine may, in a sense, be said to do things, but
what it does is of no benefit to it — does not prolong its
existence or. propagate its kind. A living being, on the
other hand, is always in action for the good of itself and its
race— removing from, danger, fighting or taking other
action against enemies, seeking and consuming food,
growing, producing young, and providing in various ways
for their welfare. The process by which the living thing
thus fends for itself and its kind is its life. In animals the
most conspicuous part of the process is the movement which
it involves ; in plants, the other great kingdom in the realm
of living beings, growth is more prominent ; but in both
many other activities, physical and chemical, are involved ;
and in both the life of individuals leads up to reproduction.

Reproduction the perpetuation of the race — is indeed
the end to which all life, tends. Life does not consist in
disconnected efforts, but is an orderly system of processes.
Movements are undertaken, or roots and leaves thrown out,
to obtain food, hood (in excess of what is used in repairing
the wasting caused, as we shall see, by movement and other
work) provides for growth ; growth for the breaking away
of a part of the body, as seeds or eggs or free buds to
become the. next generation. And all the time unfavour-
able conditions, of cold or heat, drought or wetness, the
absence of food, or the presence of enemies, must be
avoided if the process is not to be brought to an end by
the destruction of the individual.

Life, like all other processes that go on in material
bodies, requires energy. It differs from some such pro-
cesses m the way in which its energy is obtained, and from
all of them m the way in which its energv is directed.

The Energy 7h-v ene,rgy of the Process which' goes on in a

Life, boiling kettle is imparted to it continually from
without by the fire. The energy of the pro-
cess which goes on in a clock is stored in it, but has
been imparted to it by mechanical action from without
in the winding of the spring. The energy of the processes
which go on in a mass of radium, or in a cartridge which
explodes, is derived from the disintegration of part of the
substance of the radium or the cartridge itself. The
energy of the life of a living being is likewise derived

INTRODUCTORY : THE ANIMAL ORGANISM 3

from the disintegration of substances in itself. But life
differs from the other processes, and from all processes
that go on in lifeless things, in that its energy is liberated
in such a way as to tend to the preservation and increase
of the thing in which it goes on — that is, of the living
being.

The mode in which the living body avails itself of
energy contained in its own substance depends upon the
following facts. Whenever atoms unite to form molecules,
energy is set free, and the stabler the molecules formed,
the greater, almost invariably, is the amount of energy
liberated at their formation. The same amount of energy
must be used to break up a molecule as was set free when
it was formed. The molecules that compose the sub-
stances from which the body obtains its energy contain
carbon, hydrogen, oxygen, and sometimes nitrogen and
other elements, and are complex and relatively unstable
.and rich in energy. The body breaks down these mole-
cules so as to form smaller and more stable molecules.
The energy which is freed in the formation of the stabler
molecules is so much greater than that which is required
to break down those that are less stable that a large
balance of energy is set free, and becomes available for
the work of life. Usually the breaking-down process is
continued until the carbon and hydrogen atoms are in
the very small and stable molecules of carbon dioxide and
water — that is to say, it is a complete oxidation. But it is
not always so. The substances which are broken down
never contain enough oxygen to combine with all the car-
bon and hydrogen in their molecules, and therefore many
.animals and plants which live in surroundings from which
they cannot obtain additional oxygen are unable to com-
plete the process of disintegration. Thus the fungus
known as Yeast, living in solutions which contain no
dissolved oxygen, breaks down the sugar glucose accord-
ing to the following equation :

C6H120r=2C2H60 + 2C02

(Glucose) (Alcohol)

leaving, as an unoxidised residue, the alcohol, of whose
production man avails himself in brewing. Similarly,

4

MANUAL OF ELEMENTARY ZOOLOGY

many animals which are internal parasites, or otherwise
live in situations which lack free oxygen, break down the
substance glycogen (which is related to starch) so as to
form lactic acid, according to the equation :

(C6 H10O5) n -f n H20 = 2 n C3H6 03

(Glycogen) (Lactic acid)

Animals and plants which carry out such processes as
these are said to be anaerobic. Their mode of obtaining
energy, since it leaves a residue containing energy of
which they have not availed themselves, is wasteful as
compared with that of the majority of living beings, which
are aerobic , that is, draw from their surroundings — air or
water (see p. 5) — free oxygen, and with it complete the
oxidation of the substances from which they obtain their
energy. The obtaining of free energy by the disinte-
gration of complex substances is familiar to us in various
processes employed by man. Thus the energy imparted
to a bullet by an explosive is liberated, like the energy
of anaerobic animals and plants, by a decomposition
without importing oxygen, while the energy of a petrol
or steam engine or the light of a candle is obtained by
the use of oxygen from the air in combustion, like the
energy of aerobic beings.

It is not difficult to prove that this disintegration is
taking place in the body. The breaking down of the
molecules which are destroyed forms, as we have seen,
carbon dioxide and water. Since many of the disinte-
grated molecules contain nitrogen, there are formed also
certain fairly simple nitrogenous compounds, such as
urea, CO(NH2)2. (1) The intake of oxygen and loss of

carbon dioxide during life are easily demonstrated. Men
or animals enclosed in a vessel to which air has not access
are unable to live for more than a short time. The
animals are stifled, just as a fire or the flame of a candle
may be stifled, by want of air, and subsequent examina-
tion of the gases in the vessel will show that the oxygen
has been depleted and replaced by carbon dioxide, just as
it would be if a candle had been burnt in it. This loss
of carbon dioxide and the intake of oxygen which usually
accompanies it are characteristic of living animals and are

INTRODUCTORY : THE ANIMAL ORGANISM 5

known as Respiration T In man and animals like him,
they take place through, the lungs, in breathing. If the
breath be tested, it will be found to have undergone the
same changes as the air in a vessel in which an animal has
been stifled. Fishes and other aquatic animals use the
oxygen which is held in solution in the water in which
they live. They usually respire by means of structures
known as gills, which offer to the water a large surface
upon which gases can be exchanged ; of these we shall
consider examples when we study the crayfish and dog-
fish. The necessity for renewing by aeration the dis-
solved oxygen in the water of an aquarium is due to the
respiration of the inhabitants. (2) The nitrogenous waste
matters may be identified by chemical analysis in excreta
such as the urine of man. (3) The formation of water is
less easily demonstrated, because the bulk of the water lost
to the body has been taken in as such through the mouth
to perform certain indispensable functions, one of which
is the washing out of nitrogenous waste substances, which
are harmful, but a careful comparison of the quantities of
water which enter and leave the body shows that more
goes out than has entered.

While the chemical processes by which energy is
liberated in the body are all of the general character
which we have just outlined, they are nevertheless varied
in detail and extremely complicated. Oxidation takes
place, not by single reactions between oxygen and the
substances which are ultimately oxidised, but by chains
of reactions. These cannot here be described
weafS'iear, niore fully. It must, however, be mentioned
that, besides those processes in which by the
disintegration of certain substances energy is liberated,
there is involved a considerable loss of material by wear
and tear of the more permanent part of the living matter
in which the oxidised substances are contained.

The energy freed in the disintegration of the body-

1 The student is apt to be puzzled by the fact that this term is
used in different senses. In modern Botany it denotes the oxidations
by which the energy of the processes of life is liberated. In Medicine
and Zoology it retains its original meaning, and is applied to that
exchange of gases with the surroundings which is necessitated by
the oxidations in the body.

6

MANUAL OF ELEMENTARY ZOOLOGY

substance appears, as we have seen, in various processes.
The most characteristic and important of these are
contraction, chemical work, excretion, secre-
ofThe ranCC tion> and the conduction of impulses. Con-

'iberated traction is the process by which mechanical

various forms, movements are carried out. In it a portion of
the living substance changes in shape but not
m size, growing shorter in one direction but thicker in
others. This may easily be felt in the working of any of
the great muscles of the human body, as when the well-
known “ biceps,” in shortening to pull up the forearm,
grows at the same time thicker. Instances of chemical
activity, are seen in the lormation of the constituents of the
many juices which are used for various purposes in the
body. Thus the “ gastric juice,” by which food is digested
and disinfected in the stomach, contains among other
substances hydrochloric acid, whose formation in face of
the alkalinity ol the blood involves very considerable
chemical work. Other examples of liquids formed for
special purposes are the spittle or saliva which helps in the
swallowing and digestion of food, tears which wash clean
the surface ol the eyes, and so forth. The regions in which
materials are thus formed are known as glands. Again,
a part of the energy liberated in the body is used in the
discharge of materials from the substance of the bodv.
We have seen that in the process ol disintegration there
arise waste products ol which the body gets rid ; with
these it casts out poisonous or excessive materials absorbed
Irom. the food. We have just seen also that certain
activities of the body consist in the chemical manufacture
ol materials which are not purely waste but have their
uses to the body. The casting out from the substance of
the glands of the materials ol these two classes, and of the
water in which they are dissolved, is a necessary part of
the working of the bodily machine. The harmful or ex-
cessive products are got rid of because they are injurious,,
and the products of chemical manufacture are removed in
order to be of use elsewhere. Both kinds of material are
accordingly shed, sometimes upon the surface of the body,
but usually into tubes known as ducts , in which they
flow to the required locality. This shedding out is a

INTRODUCTORY : THE ANIMAL ORGANISM 7

distinct process, carried on by an exercise of the activity
of the living substance ot the body. No real distinction
can be drawn between the two cases, but the process is
called excretion when the substances cast out are purely
waste, as in the urine, and secretion when they are ot some
further use to the body, as in the gastric juice. Finally,
an expenditure of energy is involved in the conveyance
of impulses which bring about events in the body from the
localities where the impulses are started by stimuli (p. 8)
to the localities in which the events take place. Thus,
when a drop of water which has fallen upon the skin is
brushed off, an impulse is started in the skin and conveyed
along those tracts of the body which we know as nerves
till it causes such movements of the muscles of the arms
as are necessary to brush off the drop. This property in
living matter of conveying impulses is known as con-
ductivity, and it involves the evolution of energy by
disintegration in the conducting substance.

It should be noted that the forms in which the energy ot
the body is used in these and other processes are very
different. Besides mechanical movement, the exhibition
of molar energy, it may bring about chemical changes, or
become heat , as is shown by its warming the human body,
or light , as in the glow-worm, or electricity , as in the well-
known electric eel, and less conspicuously in many events
in the human and other living bodies ; and there are other
processes, such as secretion, its action in which has not yet
been certainly compared with any event in the lifeless
world.

That all this expenditure of energy is so directed as to
be of service to the creature in which it takes place, is
due to two properties of the living being which
are known as Irritability and Automatism .
These properties reside, of course, in the living substance
(protoplasm) which usually forms only a part of a. body
which is alive. Irritability is the property of acting in
response to external events. T hus, in various animals,
the coming into sight of an enemy will cause action in the
form of flight or preparation for defence ; the smell of
food sets the mouth watering ; a sound will wake a sleeper ,
and so forth. The events external to a living being which

8 MANUAL OF ELEMENTAL Y ZOOLOGY

directly affect it so that it responds by action are known as
stimuli. I wo things must be noted concerning the
activity started by them. Firstly, that its energy is derived,
as we have seen, not from without, as when a change is
brought about in water by heating it, but from within, as
when a change is brought about in gunpowder by heating
it. Secondly, that the extent of the internal change bears
no relation to that of the external one which acts as its
stimulus. Thus, when, in response to a command, a man
lilts a heavy load, the energy of the sound-waves which
call forth this reaction is immeasurably smaller than that
ot the work done by the man, and either may be greater or
less without a corresponding alteration in the other.

Activity, however, is not necessarily associated with
Automatism. irritability. The activity of living animals is
characterised by a teature which is sometimes
held to be in its essence the very opposite of irritability.
This feature is called automatism. Automatism is the
occurrence in the body of activity which is not the direct
result of any stimulus from without. The simplest in-
stance ot this is the beating of the heart. In view of the
great number of stimuli which the body is always re-
ceiving, and of the fact that one internal event can act as
a stimulus to bring about another, it is necessary to be
cautious in attributing an automatic character to any
action, but much activity is at least not due to any stimuli
that we can trace, and numerous happenings which follow
stimuli are rather the modifying of processes that are
already going on than the initiation of entirely new action.
Indeed the response to external events which is so con-
spicuous in the living being is set against a background
ol automatic procedure. In any case, the processes that
we are next to study are not direct responses to external
events.

It is, as we have seen, characteristic of , the living body

incorporation to be continually wasting its substance by
of Food : disintegration in producing energy for its

and mgesti^n. activity. Clearly this could not go on in-
definitely without some compensating repair,
d he waste is made good by the incorporation of food.
Two distinct processes may be recognised in incorporation

INTRODUCTORY : THE ANIMAL ORGANISM 9

—absorption and assimilation. Before it can be absorbed
the food ot animals has generally to undergo a preliminary
process of digestion , whereby solid or indiffusible nutri-
ment which it contains is made soluble and diffusible,
d he food must always contain the following materials :
(1) water , which is of the highest importance both as an
essential constituent of the living matter (protoplasm) and
also because it is used in the body for transporting sub-
stances in solution, as in the blood and urine, (2) certain
inorganic salts , such as the chlorides and phosphates of
sodium, potassium, and calcium, (3) the very complex
compounds known as proteins. A protein is a colloid
substance consisting ol carbon, hydrogen, nitrogen, and
oxygen, with small quantities of sulphur and sometimes
phosphorus. A familiar example is the “ albumin ”
which, mixed with water, forms white of egg. Proteins
are very complex linkages of amino-acids.

An amino-acid is a compound which contains both the basic radicle
NH2 and the acid group COOH. A simple example is Amino-
acetic acid or Glycine, CH2.NH2.COOH.

Thus in the complicated chemistry of the body proteins
are able to exercise the power, which amino-acids have,
■of uniting either with acids or with bases ; and on final
disintegration they always yield their nitrogen in a form
related to ammonia. The proteins of the body are many,
and even those ol similar parts in different animals are
slightly different. That the food does not consist of pro-
teins identical with those ol the body it is entering does not
matter, since in digestion proteins are resolved into the
amino-acids of which they are composed and the animal
so recombines these as to meet its own needs. The food
must, however, supply the right amino-acids in sufficient
quantities. It is found, for instance, that mice fed upon
a diet in which the only protein present is zein, the pro-
tein of maize, which does not contain the important
constituent tryptophane, are unable to support life. Pro-
teins are the one indispensable organic item in the food of
all animals, because, while, like other substances that we
shall mention below, they can be oxidised to provide
energy, it is they alone that can make good the protein
matter that every living body contains and loses by wear

IO

MANUAL OF ELEMENTARY ZOOLOGY

and tear (p. 5) and also that can provide such material
tor growth. When they are used for fuel the nitrogen is
discharged from their molecules as ammonia. This is
deamination ; it is the ultimate source of most of the
nitrogenous compounds which we mentioned above as
forming part ot the excreta. Besides these substances the
food usually contains (4) carbohydrates (sugars, starches,
and related substances), (5) fats. It is chiefly these two
classes of substances that are oxidised to provide energy.
Both contain carbon, hydrogen, and oxygen. In car-
bohydrates the oxygen is present in exactly the proportions
to oxidise the hydrogen, as in cane sugar and malt sugar
or maltose, which both have the formula C12H22On,
grape sugar or glucose, C6H1206, and starch (C6H10O5)«.
In fats there is relatively less oxygen ; therefore they
require for complete combustion more of that element
than is needed to oxidise the carbon, and their potential
energy is greater than that of carbohydrates. In digestion,
insoluble carbohydrates, such as starch, are dissolved by
conversion into glucose, and fats are split into soluble
components — fatty acids and glycerine.

Both these processes, and also the digestion of protein, are hy-
drolyses— decompositions into smaller molecules with the aid of water
taken up. Thus :

2(b6H10O5)« + ;/H2O = «C12H22Ou

(Starch) (Maltose)

c12h22ou+h2o=2C6h12o6

(Maltose) (Glucose)

and again :

(C1,HS5COO)3C3H5+3H02 = 3C1IH,5COOH + C3sH5(OH)8

(the tat Stearin) (Stearic acid) (Glycerine)

They are all brought about by organic catalysts (enzymes, p. 61).
Starches and proteins pass through several intermediate stages before
their digestion is completed.

Since proteins, carbohydrates, and fats are among the
compounds known as “ organic,” which, in nature, are
found only in the bodies of plants and animals and in their
remains, all animals require for food such bodies. From
these some if not all animals must also obtain (6) traces
of the mysterious vitamins. These substances, originally
manufactured by plants, are transmitted to herbivorous,
and so to carnivorous, animals, and though in very small

INTRODUCTORY : THE ANIMAL ORGANISM

1 1

quantities, are essential to life. The nature of some of
them and the mode of action of all are unknown, but they
may be recognised by their effects. Thus two at least of
them (vitamins A and B) must be present for normal
growth to take place in the animals in which they have
been investigated. When, for instance, young rats are
fed upon an artificial
liquid containing the pro-
tein, sugar, and fat of
milk in the usual propor-
tions, they fail to grow,
but the addition to their
diet of a very small
quantity of fresh milk
(which contains the vita-
mins) causes them to grow
in a normal manner (Fig.
i). The absence of vita-
min B from the diet of
adults causes inflamma-
tion of the nerves and
other disorders, which
may be removed by add-
ing to the diet wheat-
germ, yeast, or other

materials which contain FiG. i. — Curves showing the effect
the vitamin. About a of vitamins on the growth oi
dozen vitamins are known rats* — From Hopkins,

at present.

The digested materials
undergo absorption into
the substance of the body,
leaving the indigestible

matter to be cast away as the dung or fceces. Incorpora-
te Absorption tiorb howeveb is not brought about simply
and Assimifa- by the absorption of digested matter. Neither
before nor after digestion is the food of the
same composition as the substance to which it is to-
be added. The flesh of a dead ox or sheep differs con-
siderably in composition from that of a living man, and
the difference is increased by its digestion. In the course

Lower curve (white circles), rats fed on.
artificial milk alone. Upper curve (black
circles), rats fed on artificial milk and 2
c.c. of cow’s milk daily. Average weight
in grams, vertical. Time in days, hori-
zontal.

12

MANUAL OF ELEMENTARY ZOOLOGY

of incorporation the food has therefore to undergo
chemical changes by which it is converted into the
substances which compose the bodv, and these changes
it undergoes by the activity of the living matter itself.
I hat is to say, the living substance has the power of
making, out ot unlike materials, additional matter of its
own composition. The process by which this is done is
known as assimilation. Both absorption and assimilation
are processes in which work is done, and therefore in-
volve the use of energy, but their net result is to add to
the amount of material composed of complex molecules,
and therefore to the amount of energy, in the body.

It will be seen that disintegration and its complementary
Metabolism. assimilation constitute a series of chemical
changes, continually taking place in the body,
whereby there is kept up a continual evolution of energy.
These changes, regarded as a whole, are known as
metabolism , the disintegrative changes being known as
katabolism and the assimilative as anabolism.

The reactions which take place in the living body are varied and
immensely complicated. Practically all of them, however, belong to
one or other of the following classes : (i) hydrolysis (p. io) and
dehydration, (2) deamination (p. 10 ; this is probably not a simple
reaction), (3) reaction between an acid and a salt, (4)' oxidation and
reduction. Though the changes which fall into the first three of
these classes are very important parts of the process as a whole, the
liberation ot energy during them is small, and it is by reactions of
the fourth class that nearly all the energy of life is set free.

Part of the new complex material is used in the repair
_ ot the waste caused by disintegration, but the

Growth. incorporation ot new material has a further
effect than the mere repair of waste. Through-
out the body of a young animal, and in such parts as the
roots ot the hair and nails even in age, incorporation
takes place in excess of waste, so that growth occurs.
Both in repair and in growth the new material is not
added in layers to a surface, like that which is taken up
by a crystal, but is placed between the existing particles,
as a substance is taken into solution. Growth, more-
over, is a very complex architectural process in which the
nitric ate structure ot the body is built up out ot many
materials.

INTRODUCTORY : THE ANIMAL ORGANISM

13

. . > bWlp. M

v.tGmLa ' / a, 'gpfii-': ■ ;
fi Ay yff -fifiAlfiffd' A >v
fiA;. " '/ ■■ AvgA

GAN*

■ ' . v:

- .-- '■ j ' •

■ ' /■ ■ v ^iV-

Growth is followed, sooner or later, by reproduction.

That is to say, a portion of the body breaks
tion.r0dUC~ off to form a new individual which leads an
independent existence. It will be convenient
to use the word “ fission ” to denote the actual breaking
away of the new body, for reproduction is more than a
mere act of separation. This will be seen if we consider
it a little more closely.

(a) As has been said, reproduction always involves the
fission of an existing body. Life never starts anew, but is
always passed on from
one living being to
another which arises

from it. A living being
which divides to pro-
duce others is a
parent ; those which
it forms are offspring.

(ff) T he offspring
are always at first
unlike the parent.

There are, as we shall
see, certain creatures
in which the only evi-
dent difference be-
tween the offspring
and the individual by
whose division they
arose is the necessary
one of size. But in

the great majority of cases there is also an obvious
difference in form, the offspring being at first very unlike
the parent in structure. This difference is obscured in
the case of man and some other animals, where the
offspring (Fig. 2) undergoes changes in the womb before
birth, but it is seen unmistakably in animals which are
born in the condition of an egg. In their immature
condition the offspring are known as reproductive bodies.

{c) In spite of this unlikeness at starting, the offspring
become in time like the parent from, which they arose ,
owing to a succession of changes which is sometimes

- d - AhA'AFV Tv
C - r. - 1 ^ ff ■

• . • ,

• . 7

■ ,.v;- ~

■ ‘ V * . m ; .

Fig. 2. — The egg or “ ovum” from which
a human being is developed, highly
magnified.

i4

MANUAL OF ELEMENTARY ZOOLOGY

straightforward, or direct ; sometimes, as in the well-
known case of the butterfly, very roundabout, or indirect.
Thus the life of an animal or plant is a cycle , in which it
passes through a series of stages, beginning with the small
and simple reproductive body, and ending with the larger
and. usually more complex adult , ready to undergo fission
again. Every individual goes through the same cycle of
changes as its parent, resembling in each stage a similar
stage passed through by the latter, till it reaches the likeness
of the individual that produced it. This is due to the
property known as heredity. Thus, in the strict sense of
the \void, reproduction includes the whole life cycle and
consists of two distinct processes — -fissio?i and the develop-
ment of the reproductive body into the adult— for until

: Adul.t ^fis cycle- has been coin-

^ ^ pleted the parent is not

reproduced. 1 From this
I o point of view, growth is

^at part of the process of
T development by which the

D n IN reproductive body reaches

Reproductive Boor * the size of the hull. At

Fig. 3.— A diagram of the life-cycle the same time in most

of an animal, cases, and ^

, the growing individual is

undergoing the changes in structure to which we have
alluded.

Here must be mentioned a process which, though in
Syngamy, itself it is not reproductive, is closely connected
. Wlth reproduction. It is well known that in
most animals reproduction is only possible by the co-
operation. of two individuals of different kinds known
as the sexes.. 1 his is because in such animals the repro-
ductive bodies are of two sorts, each produced only by
one of the sexes, and neither sort can develop except after
fusion with one of the other sort. That fusion is an example
of the process known as a syngamy , union of two distinct
living bodies, . which occurs from time to time in nearly all
species of animals and plants. The bodies which unite

1 Development may partly take place before fission, as in many
cases of budding (Chap. XII.). y

INTRODUCTORY : THE ANIMAL ORGANISM 15

are known as gametes , and that which results from their
fusion as a zygote. In some of the smallest living beings
(Fig. 4) syngamy is the union ot fully-grown adults and
has no connection with reproduction. In other such
creatures (Fig. 10), however, and in all large and complex
animals and plants, syngamy takes place only between
the reproductive bodies, which are
generally unable to develop without
it, so that it becomes a necessary
part of the reproductive process.

In these cases the reproductive
bodies are of a kind known as
germs , distinguished from other
reproductive bodies (free buds,
etc.) by their small size and the
simplicity of their structure. The
germs of such creatures are usually
•of two sizes which unite larger with
smaller (Fig. 10 C). In all large
and complex animals (and in some
•of the smallest) the gametes differ
in form and behaviour as well as in
size (Figs. 2 and 5). One is larger
and passive, and is called the
female gamete, or, in large animals,
the egg or ovum. The other is
smaller and active, and known as
the male gamete or spermatozoon ;
it has usually a tail (flagellum) with
which it swims in the fluid in which
it is borne, and thus it moves to the
•egg and enters the latter (Fig. 6).

This process is known as the fer-
tilisation of the ovum. After it the
fertilised ovum or oosperm proceeds to develop. Ova and
spermatozoa are usually formed by different adults, known
respectively as female and male., but in some cases both kinds
are formed by one individual, which is then known as a her-
maphrodite. In some aquatic animals the gametes are set
free, and syngamy takes place outside the body of the
parent. In many cases, however, the ova are kept within

Fig. 4. — Copromonas , a
minute inhabitant of
dung. — After Dobell.

a, Adult individual ; b, the same
in fission ; c, two adult in-
dividuals in syngamy ;
d, the zygote, enclosed in a
cyst.

f.v., Food vacuole; fl., flagellum;
g., gullet; nu., nucleus; res.,
reservoir of contractile
vacuole (see p. 140).

i6

MANUAL OF ELEMENTARY ZOOLOGY

the body of the mother, and the male gametes, known col-
lectively as the sperm , are transferred in the seminal fluid
by the male to the body of the female and there fertilise
k ova. I his transference is known as

coition. Reproduction in which syngamy
is necessary before the reproductive
bodies can develop is known as sexual
reproduction ; that in which the repro-
ductive bodies are not gametes is asexual.

As we shall see (p. 593) there is a kind of re-
production (parthenogenesis) in which a female
germ (ovum) develops without syngamy. This
kind is best regarded as an aberrant form of
sexual reproduction.

The terminology ol these processes is in some
confusion. Syngamy is the fusion of two energids
(p. 109), nucleus with nucleus and cytoplasm
with cytoplasm, though one of the two may have
little cytoplasm and possibly sometimes has
none. The union of nuclei — which is by far
more important than that of the cytoplasms—
is karyogamy ; the union of cytoplasm is plas-
mogamy. The term conjugation has been used as
synonymous with syngamy and has also been
used to designate the peculiar procedure by
which syngamy is accomplished in the Ciliata
(C hap. IX). The term copulation is used as
synonymous with coition and also to denote
syngamy in the lowest organisms (Protozoa
other than Ciliata).

Summary of
the Character-
istics of Life.

We are now in a position to sum up
the characteristic features
of the complex process
which is known as life. In
doing so we shall arrange
them in a somewhat different order
from that in which they have come
under our notice, stating successively
those that relate to the starting and
stopping of the life processes, to the
nature of these processes, and to the end to which they tend.
We have found in life the following features : 1 —

x The process of respiration is not included in the following list
because, though it is often rightly cited as a characteristic feature of

1 1G. 5. — Tl uman
spermatozoa,
seen i 1 faceandin
sideview. — More
highly magnified
than Fig. 2.

INTRODUCTORY : THE ANIMAL ORGANISM 17

1. Irritability — the starting or stopping in the body of

an activity ot its own as the result of the receipt of
a stimulus.

2. Automatism the starting or stopping of activity

without an immediate external stimulus.

3* Disintegration with liberation of energy , this energy
appearing in various processes, of which the most
conspicuous are :

i. Contraction , or change of shape,

ii. Chemical work ,

iii. Excretion and Secretion , the shedding out

from the substance of the body of chemical
products,

iv. The Conduction of the impulses which start

these processes from one part of the body
to another.

4. The Incorporation of food , which involves (a) the

Absorption of new material, (b) the conversion by
Assimilation of unlike substances into the sub-
stance of the body.

5. Purposiveness the direction of the activities of the

body towards an end which concerns itself, namely,
to its own preservation and that of its kind. This
is shown :

i. as regards the individual, in the Struggle for
Existence — the obtaining of food, and the
avoidance of overcoming of enemies and
unfavourable circumstances ;
h. as regards the race, in Reproduction — the
bringing into existence of new individuals,
which involves ( a ) the breaking off by
Fission of a part of the body, and (b) the
process of growth and structural and
chemical change known as the Develop-
ment of the part broken off.

the life of animals, it is not a simple or distinct process. It consists in
the excretion ot carbon dioxide and the taking up of oxygen, which is
not in its essence different from the incorporation of other materials,
oyngamy is excluded because it is not a universal property of living

2

i8

MANUAL OF ELEMENTARY ZOO -LOGY

Life is the only property which is peculiar to living

three beings. All of them, however, present two

Character- further characteristics which are found also in
istics •

some other objects, though only in such as owe

their existence to living beings. One of these characteristics
is the presence in them of the substance known as Proto-
plasm. This they share with things that have been alive
and are now dead. The other is the existence in them of

Organisation. This they share

Fig. 6. — The ovum of a Bat, after the
entry of the spermatozoon. —
From F. H. A. Marshall, after
van der Stricht.

o.n., nucleus of the ovum ; p.b., “ polar
bodies ” (see p. 122) ; spn., spermatozoon.

not only with dead things
but with some others,
such as machines and
human societies, that have
been made by living
beings.

The life of animals and

Structure and plants is re-

Function : fleeted in their

Organisation. stmcture< The

living body is a machine
which reacts to events in
the outer world in such a
way as to prolong its own
existence and that of its
kind. Like other machines
it consists ot a number of
parts each of which does a
particular portion of the
work of the whole. Such
parts are called organs.

Thus there are sense
organs , such as the eyes and ears, for the reception of
stimuli ; nervous organs , forming a nervous system (usually
provided with a central station such as the brain), for the
conduction ot impulses set up by these and other stimuli,
to the organs which carry out the main part of the reaction ;
locomotive organs, such as legs and wings and fins, to carrv
the body towards food or from danger ; organs of offence
and defence , such as teeth and claws, for procuring food and
resisting attack ; organs of digestion , such as the stomach
and bowels ; organs of circulation , such as the heart and
blood vessels, which distribute digested food, carry waste

INTRODUCTORY : THE ANIMAL ORGANISM

19

matters to the excretory organs , such as the kidneys, and
gases to and trom organs of respiration , such as lungs and
gills, and transport materials in general ; and organs of
reproduction. An organ may consist of subsidiary organs.
1 hus the leg is supported by skeletal organs known as
bones, moved by muscles , and served by blood vessels and
nerves. A complex of parts which work together is known
as an organism , and this name is often applied to animals

Fig. 7. — A section of dry bone magnified. The dark spaces show
where the living part of the tissue was lodged.

lac.. Spaces known as “lacunae.” Tn these lay the cells into which the
protoplasm was divided, g.s., ground substance. This is traversed by
numerous “canaliculi ” in which processes of protoplasm united the cells
into a meshwork. H.c., “Haversian canals” in which minute blood
vessels lay. The lacunae are so arranged as to divide the ground-sub-
stance into concentric layers or “lamella: ” around the Haversian canals.

and to plants, for plants are also provided with organs, and
■also alive. The provision of separate organs for particular
functions is called organisation or differentiation ; the
assignment of particular functions to separate organs
which corresponds to organisation is called, by analogy
"with the similar separation of functions in modern industry,
Ihe. division of physiological labour. Organisation exists
to a very various extent among organisms, and of two
organisms that which has the larger number of different
organs is said to be the more highly organised or more

20

MANUAL OF ELEMENTARY ZOOLOGY

highly differentiated , or simply the higher. Thus man is
a higher organism than a jelly-fish. The higher the
organism, the greater is its efficiency in coping with its
surroundings, the greater the# vicissitudes in them which
it can survive. There are also great differences in form
between the organs of animals of the same grade of
organisation. I hus a butterfly is as highly organised as
a. fish, but its organs are utterly different in form. The
differences in structure between animals correspond to
differences in their .modes of life. An animal which lives
m water has, for instance, very different organs of loco-
motion and respiration from one which lives on land ; the
sense organs of an internal parasite are much less highly
differentiated than those ot an animal which has to seek
food and avoid enemies from hour to hour ; and a car-
nivorous animal has organs for seizing and eating its food
which are different from those of one whose diet is vege-
tarian. This correspondence between organisation and
mode of life is known as adaptation.

Organisation involves more than the mere localising of
Tissues. functions -more, that is, than the existence
m the body of regions where special functions
are performed. It involves also a specialisation of each of
these regions to fit it for its special functions. This
specialisation is found partly in the shape of each oro-an,
but also largely in its texture and composition. The sub-
stance ot the body is not alike throughout, but different
portions ot it have differences in texture and chemical
composition which confer upon them different properties
Thus the outer layer of the skin is firm and hard to pene-
trate, bone is rigid, blood is fluid, the substance known as
4 connective tissue” is tough and binds other tissues
together,1 nerve has the power of conduction highly
developed, and muscle that of contraction, and so forth.
Such a portion of the body-substance with particular
properties, due to a particular texture and composition, is
known as a tissue. An organ may consist of one tissue
throughout, but is usually built up of several, upon the

This may be seen in skinning anv large animal
white material which holds down the' skin and binds
together is connective tissue.

The tough,
the muscles

INTRODUCTORY : THE ANIMAL ORGANISM 21

nature and arrangement of which its powers depend. Thus
a muscle contains, besides muscular tissue, connective
tissue to bind it together and nervous tissue to conduct
through it the impulses which cause it to contract.

The processes which go on in living organisms do not
consist solely in action upon the outer world.
Regulation. A great part Gf them js directed to keeping the

machine in condition. The needs ot the several organs
in the way of food, oxygen, and the removal of waste,
are very different, and vary from time to time with the
activity of the organ. Often, too, the activity of one
organ must be accompanied by an increase or depression
of that of some other organ, as when heavy work by the
muscles calls for a release by the liver of fuel in the form
of sugar, or in an active gland or muscle the walls ot the
blood vessels relax their contraction and so allow a better
flow of blood through the working tissue. Tasks of main-
taining temperature also vary. Again, in growth the
formation of the various parts of the body needs very strict
adjustment. In all such respects the processes of the body
are subject to regulation. This is effected in two ways, by
the two systems of communication within the body the
blood vessels or other transporting system, and the nervous
system. ( a ) Substances secreted into the blood by various
organs affect the working of other organs which they reach
in the course of the circulation. Some of these substances
are not produced ad hoc. Thus the carbon dioxide passed
into the blood by active organs as a result of the oxidation
o-oing on within them alters the degree of acidity or alkalin-
ity of the blood, and this regulates the quantity and quality
of the blood supply, the acidity causing small local blood
vessels to dilate, so that the active organs are flushed with
the blood they need, and stimulating the part ot the brain
which governs respiration, so that rapid breathing oxy gen
ates the blood and removes the excess ot carbon dioxide.
But the most remarkable instances of regulation oi this
kind are effected by the secretion in small quantities ot very
powerful special agents known as u chemical messengers
or hormones. Various organs despatch these messengers,
but the most conspicuous examples of their formation are
afforded by the ductless glands. About these we shall have

22

MANUAL OF ELEMENTARY ZOOLOGY

more to say later on (see p. 62), and one example of their
functioning must suffice here. The adrenal bodies , little
glands which he near the kidneys of backboned animals,
are, in moments of anger, fear, or other emotions which
orerun violent exertion, caused, by stimuli received through
e nerves to ischarge into the blood small quantities
oi the substance adrenalin. This is carried round in the
circulation and tunes up the body for the crisis. It increases
he flow of blood in the muscles and brain by quickening
the heart beat and constricting the blood vessels of the
viscera, augments the supply of fuel for muscular action
n causing the liver to pour sugar into the blood, and in
other ways prepares the animal for action. The passing- of
secreta into the blood instead of into tubes (ducts) to be
W( to their destination is known as internal secretion .

Th® n^vous system is set into regulative action some-
nnes by the action of the blood upon the central nervous
organ, as m the case of breathing mentioned above ; but
more often messages sent in along nerves from organs are
ranslated at the centre into outgoing messages to other
organs, whose action they regulate appropriately. By them
the contraction of muscles, the secretion of glands the
narrowing or dilatation of blood vessels, the beating of the
heart to which the pressure of the blood is due, are all

about^ and tilUS 1 16 necessarf co-ordination is brought

We have hitherto spoken of the body as though it were
Protoplasm. a n ( throughout. 1 hat, however, is rarelv the
.case. 1 he living part of the body of all
organisms is a soft, slimy substance known as protoplasm
In this the whole of the metabolism goes on. In composi-
tion, protoplasm is a solution in water of organic substances
and salts, especially characterised by the presence of
proteins. About its constituents and properties we shall
have more to say later on (p. 103). In a few cases the
protoplasm makes up the whole body, but in most it is
on y a part. All tissues contain protoplasm, but many
. ontain also a framework of other substances known as
armed material , made and secreted by the protoplasm and
serving for its support. Thus in bone (Fig. 7) there is a
groundwork, consisting largely of salts of lime, to which

INTRODUCTORY : THE ANIMAL ORGANISM 23

it owes its hardness, and this groundwork is penetrated by
a meshwork of protoplasm. In many cases, as we shall
see in a later chapter, the protoplasm is divided into minute
units known as cells 1 (Fig. 8). In each cell a small proto-
plasmic body, the nucleus , acts as a regulative organ.
Nuclei are also present in tissues and organisms whose
protoplasm is not divided into cells.

c.

ABC

Fig. 8. — Portions of animal tissues, highly magnified, to

show cells.

*

A, The lining of an artery ; B, muscular tissue from the wall of the intestine ;

C, the lining of the intestine. A and B are shown in surface view, C
in section.

c., Cells; g. s ground or intercellular substanee, traversed by threads of
protoplasm from cell to cell.

We have now to observe what are the differences between
animals and the members of the other principal division
of living beings, the plants. There is no fundamental
difference in the composition of the protoplasm which is
the essential part of all living things. Nor do they differ
in the essentials of their life. This will be seen if we com-
pare instances of the activities of plants with those which
in the foregoing paragraphs we have drawn from the lives

1 For the definition of this term, see pp. 103, 109.

24 MANUAL OF ELEMENTARY ZOOLOGY

of animals. That the protoplasm of plants is irritable we
see in such cases as the turning of a sunflower towards the
sun, or the stimulation by gravity of the stem to grow
upward and the root downward, or the folding of the leaves
of the. Sensitive Plant {Mimosa) when they are touched.
That it is automatic appears in such facts as the slow
turning . of the tendrils of climbing plants till they meet
with, objects to which they can cling. That it has con-
ductivity can be seen when a stimulus given to the leaf of
a mimosa causes distant leaflets to fold. That it can
execute movements may in many cases be seen under the
microscope, when it will be found to stream round the
cell. I hat it makes substances by chemical activity and
secretes them is illustrated by the long list of drugs and
other substances obtained from plants. That it grows and
reproduces need not be argued. In the sexual reproduc-
tion of. the higher (or flowering) plants, the part of the
sperm is played by the pollen, that of the ova by “ egg-
cells ” which are contained in the flowers, in organs known
as carpels.

For all this agreement in essentials, however, there are
between animals and plants distinctions which

between C6S are b°th far-reaching and obvious. We may
Animals and take our start from familiar notions on the
***an*s' subject. Any one who tried to state in words

the ideas which he had unconsciously formed of
animals and plants would probably find them to be some-
what as follows.: An animal is a being that moves and
feeds ; a plant is a green thing that grows in the earth.
Let us. examine these notions. It will be best to base our
analysis upon our definition of a plant. We find that the
information it implicitly contains is : (i) That the plant is
green, (2) that it does not swallow food, but draws nourish-
ment from the earth (the tact that it also obtains food from
the air is less generally known), (3) that it is fixed in one

place and does not move about — usually, indeed, does not
move at all.

1. The green colour of plants is due to the presence of
the substance known as chlorophyll. This is contained in
protoplasmic structures known as chloroplasts , which in
the green cells of the higher plants are usually numerous

INTRODUCTORY : THE ANIMAL ORGANISM 25

and lens-shaped (Fig. 9). Chlorophyll is a complex com-
pound of carbon, hydrogen, oxygen, and nitrogen, con-
taining in the molecule an atom of magnesium. It is

only found in those parts of
plants which are exposed to
sunlight, and is never ?ound
in animals, except in certain
cases where minute green
plants live embedded in
the transparent protoplasm
of animal bodies, as in the
green Hydra (p. 217). At
the same time it must be re-
membered that certain plants,
such as the fungi, have no
chlorophyll.

Fig. 9. — Plant cells.

A, A small portion of green tissue from a plant. B, Part of a section through a
leaf. — From Godwin.

a.s., Air spaces between the cells ; ch., chloroplasts ; c.w., cell wall ; cu., cuticle ;
ep., epidermis surface layer of cells ; nu., nucleus ; ppm., protoplasm ; st.,
stoma (opening through which air enters) ; vac., vacuole (space containing
fluid). The arrows show the paths of diffusion of carbon dioxide.

2. More important than the mere presence of chlorophyll
is its function in the body, which is connected with the
nutrition of the plant. This function is the obtaining of
carbon from carbon dioxide by means of the energy of

26 MANUAL OF ELEMENTARY ZOOLOGY

the sun s rays, and the use of it in the manufacture of com-
plex organic substances. Absorbing certain rays of light,
the chlorophyll, in some way not yet understood, enables the

Fi<

10.

- Chlamydomonas , a minute, motile plant.

A' °s^Sin^dUcV®Tw’ Tr StageSJn *he .conjugation of gametes of equal
'/ ? - ' ’ u^y ^ y ^wo stages in the conjugation of gametes of riiffprpnt

s.zes (amsogamy). The conjugation is “ head on ’’ m each case dl«erent

C-v ■’ .VOIfltrac,t)lle vacuoles ; fh-> chloroplast ; cu„ cuticle of cellulose e eve snot •
ft-, flagellum; nu., nucleus; nu'., nuclei of two gametes about to fuse • L/

f.arc”gla(if,sPr0tOPlaSmiC ^ is in starch formataj; $£

protoplasm to use the energy of the ravs in breaking un
molecules ot carbon dioxide taken in from the surroundings
—by land plants from the air, by water plants from the
water, and by the green bodies ot Hydra from the proto-

INTRODUCTORY : THE ANIMAL ORGANISM 27

plasm of the animal. The carbon thus obtained is com-
bined with the hydrogen and oxygen of water also absorbed
to form sugar.1 This process is known as photosynthesis.
The oxygen of the carbon dioxide is set free. This can
easily be shown in the case of water plants, from whose
leaves in sunlight a stream of fine bubbles of oxygen may
be seen to ascend. The sugar is used on the one hand for
the manufacture of the more complex carbohydrates, such
as starch, in which the plant body is usually very rich,
and on the other hand for the formation of the various
substances which the protoplasm of plants, like that of
animals, requires for food, and in particular of proteins.
The nitrogen, sulphur, and phosphorus for this purpose
are obtained by the plants as salts in solution in the water
which is taken in by their roots, or sometimes, as in sea-
weeds, by the whole surface of the body. From this
peculiarity of nutrition arise several other features peculiar
to the life of plants, (i) We have here the reason for the
well-known fact that green plants cannot live in the dark,
(ii) While animals, as we have seen, are always taking in
oxygen and giving out carbon dioxide, green plants in the
light are continually taking in carbon dioxide and giving
out oxygen. Yet it must be remembered that the proto-
plasm of plants undergoes continually a true respiration
like that of animals although this is obscured by the
reverse process taking place to a greater extent during
daylight, (iii) Though the food supplied to the protoplasm
is similar m the two kinds of organisms, plants manufacture
its organic components from simple substances, whereas
animals obtain them from other organisms. Therefore,
while the food of animals consists of complex organic
substances, usually in the state of a solid or the viscous
liquid protoplasm, and has to be swallowed through an
opening, the materials taken in by green plants are simple
inorganic substances which can be absorbed as gases or
liquids through the surface of the body. It must be
noticed, however, that plants which have no chlorophyll,
such as fungi, and some animals which live as parasites
or in decaying matter, absorb their nourishment through

1 Probably carbonic acid, H2C03, is reduced to formaldehyde,
H.COH, which then polymerises into sugar.

28

MANUAL OF ELEMENTARY ZOOLOGY

the surface of the body, but take it in the form of organic
substances, more or less complex in various cases, from
the living or dead bodies of other organisms.

F^om the mode of nutrition of plants there follows the
third character which we have marked in them. In the
great majority ot animals food must be either sought by
ocomotion or at least seized by other active movements , as
it is, tor instance, in a sea-anemone or Hydra (Chap. XII )
In plants, on the other hand, not only is this necessity
absent, but, since it is desirable that they should expose as

great a surface as possible to air and water for absorption

as they do, for example, in leaves and roots— the shape of
their bodies is necessarily such as to be an actual hindrance
to motion. Thus in most plants active motion is restricted
or absent, and muscular and nervous tissues are not found
m plant bodies. Certain microscopic aquatic organisms
however— members of the group known as Flagellata—zxz
exceptions to the rule that locomotion accompanies the
animal mode of nutrition only. Though they have one or
more chloroplasts and nourish themselves like plants, their
body (which is so small that it has only one nucleus) is
compact, shaped like an egg or a spindle, and possesses
one or two fine lashes of protoplasm (flagella), bv the
working of which it is rowed or drawn through the water,
m search ot sunlight of the needed strength. Many of
them, including the example, C hlamy domonas , shown in
lug. io, have a pigment spot which is a sense organ for
the necessary appreciation ot light, whose rays it absorbs.
It is in such organisms as this that plant and animal meet.

. 4- 4 Fe necessity tor surface leads to a lourth character
in plants. An extensive surface needs strong support In
correspondence with this need we find in plants a massive
skeleton which forms a strong wall to each cell, so that the
protoplasm is upheld by an intricate framework of com-
partments whose walls are thickest in the most woody
parts ot the body. Owing, no doubt, to the ample supply
ot starch at the command of the plant, this skeleton consists
of a modified form of starch known as cellulose . Among
plants, even including those like the fungi which have no
chlorophyll, cellulose is almost invariably present ; among
animals it is unknown. It happens, indeed that this con>

INTRODUCTORY : THE ANIMAL ORGANISM 29

paratively unimportant character comes nearer than any
other to giving an absolute distinction between the two
kinds of organisms.

To sum up : we find between typical plants and typical
animals the following distinctions : —

1. The presence in typical plants and not in animals of
the green substance chlorophyll.

2. That while plants absorb through their surface simple
inorganic compounds arid from them manufacture food-
stuffs for their protoplasm, animals swallow the com-
plex substances of the bodies ot plants and ol other
animals.

3. That while in all the familiar plants motion is re-
stricted or absent, in animals it is conspicuous.

4. That plants have a skeleton of cellulose , which is
absent from the bodies of animals.

The difference in nutrition between animals and plants
has the important result that in their action

The Basance UDOn the inorganic world these two kinds of

of Nature. j , P , , •,

organisms bring about precisely opposite

changes, and do so in such a way that each sets up con-
ditions favourable to the activity ot the other. I he plant,
absorbing the energy of the sun’s rays, builds up with
storage of that energy complex organic compounds from
simple inorganic substances.1 These manufactured sub-
stances it assimilates, partly in repairing the waste of its
protoplasm, but mainly in adding to its substance by
growth. Its construction of organic materials is in excess
of its destruction ot them, and the net result ol its activity
is to provide an accumulation of those complex substances
which form a necessary part ol the lood of protoplasm.
At the same time it sets free oxygen. The animal, on the
other hand, obtains the organic food lor its protoplasm in
substances manufactured by plants, taking them either
directly from plant bodies or after they have been in-
corporated in a somewhat altered form into the proto-
plasm of other animals. In the protoplasm ol the animal
these substances undergo destruction, in consequence of

1 The storage of energy is, of course, due to the fact that more is
taken up in splitting the stable inorganic molecules (p. 26) than is
freed in forming the unstable organic molecules.

30

MANUAL OF ELEMENTARY ZOOLOGY

which there are set free carbon dioxide and simple nitrogen
compounds. Thus plants provide food and oxygen for
animals, while animals, destroying this food, provide simple

nitrogen compounds 1 and carbon dioxide for the use of
plants.

Biology comprises Botany, which deals with plants,

Zoology- alK' Zoology, which deals with animals. Now
iPian of study. tin organism niay be regarded from two points
of view according as attention is concentrated
upon its structure or its functions, though of course these
two are so closely connected that it is impossible to study
structure intelligently or function at all without reference
to the sister topic. . The sciences of Zoology and Botanv are
correspondingly divided each into two subordinate sciences,
Anatomy or Morphology , which deals with the structure
of the bodies of organisms, and Physiology , which deals
with their functions. In the following pages we shall of
necessity approach Zoology in the first place from its
anatomical side, but shall equally seek from Physiology
lght upon the meaning of the structures we find, endeavour-
ing to trace in the bodies of the particular animals studied
the provision which exists for carrying out all those functions
which our survey m this chapter has revealed to us as
taking place generally. With this purpose we shall first
examine m considerable detail one of the higher animals
then study an exceedingly simple example, and afterwards
survey, more briefly, a series of further examples. In these
there will appear two phenomena that we have already
noted (p 19) — (a) that organisation exists in many grades
•of complexity (our examples will be taken roughly in an
.ascending order in this respect), and (h) that organisation
differs not only m degree, but in kind, and that its different
kinds adapt animals to live in different surroundings, or to
take advantage of different circumstances m the same sur-
roundings. Finally, we shall discuss certain topics which con-
cern animals m general, surveying the modes in which they
reproduce their kind, and considering the relation of this
reproduction to their immense variety, and the part which
their multifarious activities play in the economy of Nature.

1 These are not available for the use of most plants till thev have
sheen turned into nitrates by the action of bacteria.

CHAPTER II

THE FROG : EXTERNAL FEATURES AND

BODY-WALL

The Common Frog of Britain is the species known in
Zoology as Rana temporaria. It is abundant
Habits‘ in summer in damp places, but in winter is less

easilv found, owing to the tact that it is then in a torpid
state, hidden in holes or buried in mud. In the spring the

FIG. ii. — ‘ The life-history of a frog. — After Brehm.

1-3 Developing ova; 4, newly hatched forms hanging to water- weeds;

’5, 6, stages with external gills; 7-10 tadpoles during emergence of
limbs ; 11, tadpoles with both pairs of limbs apparent ; 12, metamor-
phosis to frog.

warmth wakes the irogs and they congregate, croaking
loudly, and pair in the water, where the eggs are laid,
enclosed in jelly as a mass ot spawn and fertilised by the
sperm which the male sheds over them as they pass out ol
the female. In about a fortnight there hatches from each
egg a little, fish-like tadpole. 1 his has no limbs, but a
strong tail, which it uses for swimming, breathes wholly by

31

32

MANUAL OF ELEMENTARY ZOOLOGY

gills, and is at first without a mouth. A young animal
which like the tadpole differs markedly from the adults of
its kind, but is capable of fending for itself, is known as a
larva. The term e?nbryo is applied to a young organism
while it is helpless and is developing within the body of its
parent or under shelter of an egg-shell or a jelly coat, like
the young of man or a bird, or the early stages of the frog.
In a few days the tadpole comes to possess a mouth and

begins to feed on vegetable matter. Gradually it changes,
losing its gills and tail and gaining lungs and two pairs of
limbs, till at the end of three months it becomes a small
frog. Henceforward it lives principally on land, some-
times crawling about by means of both pairs of limbs,
but generally jumping with the strong hinder pair and
using the small fore pair to break its fall when it alights.
From time to time, however, it takes to the water, and
then it can swim strongly with its hind limbs. Its food,,
after it has left the water, consists of slugs, snails, insects,

FROG: EXTERNAL FEATURES AND BODY-WALL 33

worms, and other small animals, the smaller prey being
caught by a sticky tongue, the larger seized with the mouth.

In examining the body ot a irog, we are struck, first by
the fact that "its mottled green and yellow skin

Features is soft and slimy ancl without the covering of

hairs, or scales, or feathers which we find in

other animals, and next by its consisting only of head,
trunk, and two pairs of limbs. There is no neck or tail.
The trunk is flattened and bears the head at one end and
the limbs of each pair opposite to one another on the narrow
sides. In consequence of this symmetrical arrangement
we may distinguish a back or dorsal surface , a lower or
ventral surface, right and left sides, and fore and hind ends.
Such a symmetry is called bilateral , and we shall see that
in the frog it extends to the arrangement of nearly all the
organs of the body. The fore or anterior end is that which
is foremost when the animal moves, and is thus the first
part to come into relation with objects in the world around
it. At this end is placed the head , a distinct region of the
body, smaller than the trunk, which bears the mouth with
which food is taken and the three pairs of principal sense
organs by which the animal becomes aware of the nature
of its surroundings. The eyes are large, and have stout,
almost immovable upper lids and thin, translucent, movab e
lower lids.1 The nostrils or external nares are a pair of
small openings on the top of the head in front of the eyes.
Each of them leads into a chamber which communicates
with the mouth. There is no flap to the ear, but the drum
shows upon the surface at the side of the head behind anc
somewhat below the eye. If the drum be pierced, a bristle
passed through it will be found to reach the mout . n
the lower side of the trunk there may be distinguished two
regions — the large, soft- walled belly or abdomen behind,
and the smaller stout-walled breast region m front.

The limbs of each pair resemble one another, and those
of the two pairs correspond roughly in shape, each con-
sisting of three successive parts, the first two slender anc

three

1 These do not represent the lower lids of man, which are wanting
in the frog. The lower lid of the frog is the third eyelid or nictitating
membrane found in many other animals (pp. 492> 51 )•

eyelids are well developed in birds.

3

34

MANUAL OF ELEMENTARY ZOOLOGY

the third broad and adapted to be applied to the ground.
In the fore limb or arm the segment nearest the body is
known as the upper arm or brachium , the middle segment
as the forearm or antebrachium , and the third segment as
the hand or manus. In the hand may be distinguished
a zvrist or carpus , a palm or metacarpus , and fingers or
digits , of which there are only four, that which corresponds
to the thumb or pollex of man being absent. The first
finggr of the male frog bears at the breeding season a
rough-skinned swelling, not unlike the ball of the human
thumb. In the hinder limb or leg , which is longer than the
arm, the first segment is known as the thigh or femur , the

Fig, 13. — The right hand, A of a male frog at the breed-
ing season, B of a female, C of a male out of the
breeding season.

.second as the shank or crus , the third as the foot or pes.
The foot contains regions corresponding to those of the
hand, and known respectively as the ankle or tarsus , instep
or metatarsus , and toes or digits, but the ankle is much
longer than the wrist, and all five toes are present and
united by webs of skin, so that a wide surface is provided
for use in swimming. The lower side of the foot is the
Plantar surface or sole , that of the hand the palmar surface.

Between the legs at the hinder or posterior end of the
trunk is the vent or cloacal opening , through which are
passed the faeces, urine, and eggs or sperm.

The skin of the trog is a thin, tough, protective covering.

Like that of many other animals it consists of
a layer of connective tissue known as the
.dermis with a covering of cells called the epidermis.

FROG: EXTERNAL FEATURES AND BODY-WAIL

66

The dermis contains pigment cells (Figs. 6

to /

B), and

imbedded epidermal glands of several kinds which be-
tween them give a slimy liquid that possesses slightly the
acrid property found in the secretion of the skin of toads
and newts. _ The pigment in the cells is expanded and
contracted in varying conditions of light, temperature, etc.,
and thus the colour of the frog changes (p. 64). Cold’
dark, or wet surroundings cause expansion of the pig-
ment and darkening of the skin. Warmth, light, or
dryness cause contraction. From time to time the horny
outer layer of the

epidermis is shed and d.s.s. v. sP. c.

eaten by the frog.

Immediately below
the skin

l. V. c

Genera!
Arrangement
of Internal
Organs.

is a series
of large
spaces,

the subcutaneous per'
lymph sacs, contain-
ing a fluid known as
lymph (p. 75). Be-
tween the lymph sacs
the skin is bound
down to the under-
lying flesh by tough,
white connective tis-
sue, but in conse-
quence of the pre-
sence of the sacs it is
much looser than that of
sacs the body possesses
which consists, as the
does, of muscles. There

muse.

g. msnt.

coe.

Fig. 14. — A diagram of a transverse section

through the abdomen of a male fro

coe., Coelom ; d.ao., dorsal aorta ; d.s., dorsal lymph
sac ; d.s.s., dorsal subcutaneous lymph sac ; g.,
gut; i.v.c., inferior vena cava; k., kidney; l.s.s.,
lateral subcutaneous lymph sac ; msnt., mesen-
tery ; mso., mesorchium ; muse., muscular body-
wall ; n., spinal nerves ; per., peritoneum , sk.,
sp.c., spinal cord ; t., testis ; v., vertebra ;
ventral subcutaneous lymph sac.

skin .
v.s.s.,

most animals. Below the
a continuous layer ot flesh,
substance so-called always
is thus a body-wall , composed
ot skin and muscles, with bones and a lining of
peritoneum (to be mentioned shortlv), and this wall
encloses in the trunk a large space, the body cavity
or coelom, in which lie most of the principal viscera. The
latter name is applied to the soft internal organs ot
the body, such as the stomach, bowels, liver, lungs, and
heart. The viscera pretty well All the body cavity, but

36 MANUAL OF ELEMENTARY ZOOLOGY

it is easy for them to move in it because they are lubricated
by a fluid — the ccdomic fluid. The Irog is a backboned
or “ vertebrate ” animal : that is, the body- wall ol its
back, which is much thicker than that of its belly, con-
tains a structure known as the backbone , spine , or vertebra
column. This consists of a row of ring-like bones, the
vertebra , placed end to end to form a tube, the vertebral
or spinal canal, in which lies a part of the nervous
system known as the spinal marrow or cord. 1 he muscles
of the ventral side are thicker at the ends of the trunk
where they contain bony hoops, the shoulder girdle and
hip girdle, which, with the vertebrae between the upper
ends of each of them, encircle the body. The coelom is
lined bv a smooth membrane, the peritoneum, which is
continued over the viscera, so that these are not tru y
exposed in the body cavity, but hang into it in folds of
the peritoneum (Fig. 14). Each fold tits closely over the
oman which it suspends, and above the organ the tuo
sides of the fold come together to form a sheet which,
slings the organ from the body-wall. 1 he largest of these
suspensory sheets is that which holds the gut and is known
as the mesentery. Between the peritoneum and the muscles
of the back is on each side a large dorsal lymph sac, and
in each dorsal lymph sac lies one of the pair of kidneys.
In the head there is no body cavity, and the backbone 1*
here continued by a large box of bone and cartilage known as
the skull , while the spinal cord is prolonged into the skull by
the brain. The limbs 1 ave neither body cavity nor viscera,
and among their muscles lie the bones which support them.

The skeleton of the frog is composed chiefly of bone , but
contains also a good deal of a gristly substance
Skeleton: known as cartilage. There may be recognised

Arrangement, in it an axial part , consisting of the skuil,
backbone, and breastbone, which supports the
trunk and head, and an appendicular part, comprising the
bones of the limbs and their girdles, which supports the
arms and legs and anchors them to the trunk.

In the backbone there are nine vertebrae and a long
bone, known as the urostyle, which repre>ents
Backbone. several vertebrae fused together. The ninth is

known as the sacral vertebra , and to it is attached the girdle

FROG: EXTERNAL FEATURES AND BODY- WALL 37

p.m.

Fig. 15.— The skeleton of a frog, seen from above.

a., Astragalus ; br ., bristle passed into opening for last spinal nerve ; car.,
carpal or wrist bones; cl., clavicle; cm., calcaneum ; cor., coracoid,
cr., calcar; d., deltoid ridge; e.n., external narial opening ; ex. , ex-
occipital; fe. , femur; fp-, fronto-parietal; h h'., heads of humerus
and femur; hu ., humerus ; il., ilium; is., ischium; m., maxilla, m.c .,
metacarpals ; mt., metatarsals; n., nasal bone ; ol., olecranon process,
ph ., phalanges; p.m., premascilla ; pro., prootic ; fit., pterygoid; g.j. ,
quadratojugal ; r.u., radioulna; sp., sphenethmoid ; s.q., squamosal,
sup., suprascapula ; t., distal tarsals ; tf., tibiofibula ; u., urostyle,
v.i, first or atlas vertebra ; v.q, ninth or sacral vertebra.

The dotted regions consist of cartilage. The cartilage at the ends of the
limb bones is the “ articular ” cartilage which caps the enlarged ends o
the bones.

3S

MANUAL OF ELEMENTARY ZOOLOGY

of the hind limbs. Each vertebra consists of a body or
centrum and an arch, the neural arch, placed above the
centrum so as to form a ring around the spinal cord.
The hollow of each ring is a vertebral joramen, and the rings
together form the vertebral canal. The roof of each arch
is raised into a low ridge, the neural spine or spinous process,
and in every vertebra except the first the arch bears on

each side, a little above its junction with the centrum, a

transverse processr

r. c.

which is especially
large in the sacral
vertebra. At the
end of each trans-
verse process is a
small knob of cartil-
age which represents
a rib. That part of
the arch which lies
between the trans-
verse process of each
side and the neural
spine is known as
a lamina , and the
part between the
centrum and the
transverse process is
a radix or pedicle.
Each vertebra is

Fig. 16. — Vertebrae of a frog. A, fourth
vertebra, seen from in front ; B, sixth
and seventh vertebrae from the right.

az., Prezygapophysis ; cett., centrum; i.n., inter-
vertebral notch; lam., lamina of neural arch; juuuuu uw
n.a., pedicle of same; n.c., vertebral foramen*, fnr or-irl behind it
h.s., neural spine 'Med., pedicle; /a., postzyga- lr0nl akQ .UCimiU
pophysis; r.c., cartilage at end of transverse foy projections, One
process ; tr., transverse process. Qn gide &t each

end of the arch, at the junction of transverse process and
lamina, known as zygapophyses. The zygapophyses in
front of the vertebra (prczygapophyses ox superior articular
processes') have each a flat surface facing upwards. Those
behind ( postzy gapop hy ses or inferior articular processes /
have flat surfaces facing downwards which fit on to the
surfaces of the prezygapophyses and slide over them as the
backbone bends. The front and hind edges of each pedicle
are concave, forming thus intervertebral notches , and the

FROG: EXTERNAL FEATURES AND BODY- WALL

39

adjoining notches of two vertebrae form an intervertebral
foramen , through which a nerve passes from the spinal
cord. Most of the centra are hollow in front and rounded:
behind and thus fit together by ball-and-socket joints, but
the first vertebra has in front two hollows, which serve as-
sockets for two knobs, known as the occipital condyles,,
on the hinder end. of the skull, while the eighth is hollow
behind as well as in front, and the ninth projects in front,
to articulate

with the AT‘

hollow of the
eighth, and
has behind two
knobs which
articulate with
two hollows on
the urostyle.

The latter is
a long, taper-
ing bone with
a ridge above,
in the front
part of which
is a canal for ?•/
the hinder part
of the spinal
cord.

In the skull,
the following
regions may be
distinguished :

(i) the era -

nium or brain ease , (2) the nasal capsulesT

which enclose the organs of smell, (3) the
auditory capsules , which enclose the inner part of the ear,
(4) the visceral arches , an apparatus which lies below the
cranium and is highly developed in a fish and in the
tadpole, but in the adult frog is represented only by
the jaws and by a structure in the floor of the mouth
known as the hyoid.

The cranium is an oblong box, from which the nasa!

r-™,, SP-

Pi- *’ _

’ prs*
'II.
— Vi

col. pro. ex. o.c. JX.X. V.VII.

Fig. i 7- —The skull of a frog, seen from below.

col., Columella; ex., exoccipital ; t.n., internal narial open-
ing ; ot., maxilla; o.c., occipital condyle; pi., palatine;:
p.vi., premaxilla ; pro., prootic ; prs., parasphenoid ;.
pt., pterygoid ; g.j. , quadratojugal ; sp., sphenethmoid
v., vomer (prevomer) ; v.t., vomerine teeth ; II, V, VI,
VII, IX, X, foramina for cranial nerves.

nas.

4o MANUAL OF ELEMENTARY ZOOLOGY

capsules project in front and the auditory capsules at the
sides of the hinder end, while the bones of the upper jaw
form a scaffolding fixed to the capsules and supporting the
tides of the head. Between the scaffolding and the
cranium is on each side a large space known as the orbit
in which lies the eye. The hinder part of the cranium,
between the auditory capsules, is known as the occipita ,
region, the middle part, between the orbits, is known as
6 the sphenoidal re-

gion, and the front
part, immediately be-
hind the nasal cap-
sules, is known as
the ethmoidal region.

The skull consists
of a foundation of
cartilage taken over
from the tadpole

with certain bones
q.j. which are formed
while the tadpole is
q. changing into a
frog These bones

may be divided into
Fig. i 8.— The cartilaginous skull of a two sets according
frog, seen from above after the removal ^ tpe way [ n which

of most of the bones. they are formed.

a./., Anterior fontanelle \ au., auditory capsule ; cr ., 'ppggg which arise by
cranium ; ex., exoccipital ; l.pf., left posterior , r

fontanelle; nas., nasal capsule ; o.c., occipital the replacement Ot

condyle ; pro , prootic : pt., pterygoid ; q., qua * ^ fU original

rate; q.j., quadratojugal ; sp., sphenethmoid ; pariS Op LHC uiifciiiai

u.j., upper jaw bar. CartllagmOUS Skull

by bone are known as replacing or cartilage bones Those
which appear in development without being thus pre-
formed in cartilage are known as investing or membrane
bones on account of the membrane, consisting of a sort
of connective tissue, which at first occupies the places in
which they will appear. The cartilage bones are embedded
in the cartilage of the skull and cannot be remove , ut
the membrane bones can easily be taken oft.

At the hind end of the cranium is a large opening known
as the foramen magnum , through which the spinal cord is

o. c. >ex.

FROG : EXTERNAL FEATURES AND BODYAVALL 41

continuous with the brain. On each side of this is a carti-
lage bone called the exoccipital , which bears one of the
occipital co?idyles mentioned above, but the foramen
magnum is not completely bordered by the exoccipitals,
since these are separated above and below by cartilage.
The rest of the cranium is mainly composed of cartilage
covered by certain membrane bones, but the front end is
formed by a cartilage bone known as the sphenethmoid.
This has the form of a dice box divided across the narrowest
part by a transverse partition which closes the cranial cavity
in front. A longitudinal partition divides the front half of
the box into two. The roof of the cartilaginous cranium is
pierced by three large holes or f ontanelles , but these are not
seen in an intact skull, since the whole roof is covered, from
the exoccipitals to the sphenethmoid, by two long bones, the
frontoparietals , placed side by side. The floor is complete,
and under it lies a large dagger-shaped bone, the para-
sphenoid ', placed with the blade of the dagger forward
and the crosspiece of its hilt under the auditory capsules.

The wall of the cranium is pierced by certain openings
or foramina , for the passage of the nerves which arise from
the brain. These “ cranial ” nerves are ten in number on
each side. The first nerve of each side passes through a
foramen in the transverse partition of the sphenethmoid on
its way from the organ of smell in the nasal capsule. The
second-nerve , which serves the eye, enters the skull through
a conspicuous opening on each side in the middle of the
sphenoidal region. The third and fourth nerves have each
a minute foramen in the side of the same region. The fifth
and seventh nerves pass through a large common opening
on the under side of the skull, situated in a notch in the
prootic bone mentioned below. The foramen for the sixth
nerve is a small opening between those for the second and
for the fifth and seventh. The eighth nerve enters from
the inner part of the ear by an opening in the wall between
the cranium and the auditory capsule. A foramen for the
ninth and tenth ?ierves is situated in the exoccipital bone,
at the side of the occipital condyle.

The nasal 1 capsules are a pair of irregular, mainly
cartilaginous enclosures continuous with the front end of
the cranium. Only their hinder part is ossified, and this

42

MANUAL OF ELEMENTARY ZOOLOGY

foims that part of the sphenethmoid which lies in front of
the transverse partition of the latter. The wall between the
two capsules is known as the mesethmoid. Through these
capsules run the passages from the nostrils to the mouthr
and each of them has therefore an opening above and
below. Each bears two membrane bones, one on its upper
side and one beneath. The upper bone is known as the
nasal , and is shaped like the outline of a pear, with the
stalk directed outwards. The lower is the voiiier {pre-
vomer). It is of irregular shape and carries a patch of teeth
which project through the skin of the roof of the mouth.

The auditory capsules are blocks of cartilage continuous

with that of the
cranium. Each
contains a com-
plicated space,
the cartilaginous
labyrinth , which
lodges a struc-
ture known as
“ the membran-
ous labyrinth of
the ear.” Part of
the front of the
capsule is ossified
to form the
p r o o ti c bone.
Above, there abuts in its outer side a T-shaped mem-
brane bone known as the squamosal , which touches it by
one limb of the cross-piece of the T, the main limb being
directed outwards and downwards. At one spot on the
outer side of the capsule the cartilage fails and the laby-
rinth is covered only by membrane. This gap is known
as the fenestra ovalis, and from it a slender rod of bone
and cartilage, the columella auris, runs to the drum of the
ear, so that when the latter is thrown into vibrations by~
sound waves its movements are transferred by the columella
to the labyrinth through the membrane.

The framework of the upper jaw is composed of two-
series of structures, an outer, which borders the opening
of the mouth, and an inner, which supports the outer

Fig. 19.— The skull of a frog, seen from
behind.

col. , Columella; ex., exoccipital ; fm., foramen
magnum; o.c., occipital condyle; pro., prootic ;
pt., pterygoid ; q. , quadrate; q.j., quadratojugal ;
sq., squamosal ; IX. X., foramen for ninth and
tenth cranial nerves.

FROG: EXTERNAL FEATURES AND BODY-WALL 43:

e.n.

o.c.

a.c.

—ft. in..

nun..

The inner series is known as the palato-pterygo-quadratt
on account of the parts of which it is composed. These
are as follows. From the junction of the cranium with)
the nasal capsules there projects outwards a bar of carti-
lage, against the hinder, or orbital, side of which lies a.
membrane bone known as th palatine} At its outer cndi
the cartilaginous bar turns backwards, and here another
membrane bone, the pterygoid ,x fits against its inner
side. The pterygoid is Y-shaped, with the fork directed!
backwards, the inner branch of the Y abutting on the*
auditory cap-
s u 1 e. The
outer branch
underlies the
main branch
of the squa-
mosal, and
between these
two bony rods
there projects
outwards from
the auditory
capsule a rod
of cartilage,
the quadrate ,
continuous
with the longi-
tudinal bar.

With the qua-
d r a t e. h e 1 d

firm as it is by processes of the squamosal and pterygoid,
articulates the lower jaw, for which the structures in
question are said to form the suspensorium. . The
outer series of bones of the upper jaw begins with the
prem axilla , a small membrane bone applied to the
front of the nasal capsule, on to whose upper surface it
sends a process. The two premaxillse meet in the midd e
line, forming the tip of the upper jaw, and each of them,

1 The palatine and pterygoid are cartilage bones in certain of the
animals in which they occur. In the trog the cartilage bone is>
replaced in development by membrane bone.

a.sft.

Fig. 20. — The skull of a frog, seen from the
right side.

a.c.

, Anterior cornu of hyoid ; a.sft., angulo-splenial ; b.,.
body of hyoid ; col., columella ; d., dentary , e.n.,
external narial opening ; f-P-i fronto-parietal ; in .,
maxilla; min., mentomeckelian , n., nasal , o.c., occi--
pital condyle; ft.c., posterior cornu of hyoid, ft. in.,
premaxilla ; ftro., prootic ; ftt., pterygoid , q., quad-
rate ; q.j , quadratojugal ; sft , sphenethmoid ; sq.,.
squamosal.

44

MANUAL OF ELEMENTARY ZOOLOGY

bears a row of teeth. Behind the premaxilla, on each side,
another membrane bone, the maxilla , continues the edge
of the jaw. The maxilla is a long slender bone which bears
a row of teeth. Along the greater part of its length it is
supported by the nasal capsule and pterygoid, but the
hinder part lies free till it meets a small membrane bone,
the quadratojugal , which connects it with the quadrate.

The lower jaw or mandible consists of two halves united
in front by a ligament. Each half is a curved rod of
cartilage, known as MeckeV s cartilage , ossified at the end to
form the small mentomeckelian bone, and almost completely
ensheathed by a couple of membrane bones, the angulo-
splenial within, and the dent ary without. The latter
does not, as its name would imply, bear teeth, the frog
having no teeth in the lower jaw. At the near end or
angle of the jaw the dentary bears a small knob or condyle ,
which fits into a hollow, known as the mandibular fossa , on
the end of the quadrate.

The hyoid is a flat structure in the floor of the mouth. It
consists of a wide body with two short processes on each
side and two longer processes, the cornua , at each end. The
anterior cornua are very long and slender and curve back-
wards at the sides of the body and then upwards to be
attached to the sides of the auditory capsules. The posterior
cornua are shorter and stouter and project backwards at the
sides of the windpipe. They are the only ossified parts of
the hyoid, the remainder consisting of cartilage.

The following table represents in a summary form the
architecture of the skull :

Regions of skull .
Cranium

Nasal Capsules

Auditory Capsules
Visceral Arches —

Upper jaw

Cartilage bones,
f Exoccipitals
(Sphenethmoid (part)
I Sphenethmoid (part)
(Mesethmoid
Prootics

Mentomeckelians
Posterior cornua

Membrane bones.
P ronto-parietals.
Parasphenoid.
Nasals.

Vomers.

Squamosals.1

Premaxillae.

Maxillae.

Quadratojugals.

( Angulo-splenials.
1 Dentaries.

None.

Lower jaw
Hyoid

1 Attached to auditory capsules

but not belonging to them.

^ (Palatines)
^(Pterygoids

FROG : EXTERNAL FEATURES AND BODY- WALL 45

The shoulder girdle or pectoral arch is a flat structure

Limb Cirdies. cart^age and bone embedded in the body-
wall of the forepart of the trunk, which it
almost encircles. It consists of two similar halves, one
on each side of the body, united below but separate
above, where they are bound by muscles to the back-
bone. Each half is composed of an upper scapular
portion or shoulder hlade
and a lower coracoid
portion. The uppermost
part is a broad, flat plate
lying on the back known
as the suprascapula. A
great part of this consists
of cartilage stiffened by
calcareous matter, but it
has a narrow rim of plain
cartilage and a patch of
true bone 1 lies upon it,
where it joins the scapula ,
a narrower but stouter
bone placed at the side of
the body. A forward pro-
jection from this bone is known as the acromion process.
To the lower end of the scapula is attached the coracoid
portion of the girdle. This is a plate of cartilage and
bone lying on the under side of the body in the breast
region and pierced by a wide oval space called the cora-
coid fontanelle. Behind the fontanelle lies the stout
coracoid bone ; in front is a narrow strip of calcified
cartilage, the precoracoid , continuous with another strip
known as the epicoracoid which forms the inner border
of its half of the girdle and lies against its fellow in the
middle line. This junction of the two halves of the
girdle is known as its symphysis . Scapula and coracoid
are cartilage bones. A pair of slender membrane bones,
the clavicles , overlie the precoracoid cartilages. Each

1 The structure of bone has already been alluded to. It will be
more fully described together with that ot cartilage in a later chapter.
Bone differs from cartilage not in the mere presence ot calcareous
matter, but in structure and composition.

Fig. 2i.- — The hyoid apparatus
of a frog.

a.c. , Anterior cornua ; b., body;
p.c., posterior coinua.

-46

MANUAL OF ELEMENTARY ZOOLOGY

sends forward a prolongation beside the acromion process.
At the junction of the scapula and coracoid bones is the
glenoid cavity , a hollow, lined by cartilage, on the hinder
edge of the girdle, into which fits the head of the humerus
or bone of the upper arm.

To the ends of the epicoracoids, before and behind
the girdle, are attached certain structures which corre-
spond to the breastbone or sternum of other animals. In
front is a bone known as the omo sternum , bearing at its

sup.

Fig. 22.—-' The shoulder girdle of a frog, seen from below, with
the right scapula removed.

acr., Acromion process ; c.f., coracoid fontanelle ; cl., clavicle ; cor., cora-
coid ; ex., epicoracoid ; ep., episternum ; g.c., glenoid cavity ; h., head
of humerus ; om., omosternum ; p.c., precoracoid ; sc., scapula ; sup.,
suprascapula ; x., xiphisternum ; x.c., xiphoid cartilage.

•end a small plate of cartilage, the episternum. Behind is
the larger xiphisternum , bearing the broad, flat xiphoid
cartilage .

The hip girdle or pelvic arch lies at the hinder end of the
trunk in a position similar to that occupied in front by the
shoulder girdle, which it also resembles in consisting of two
halves, each composed of several pieces, joined below in a
symphysis. Its shape, however, is very difterent ; it is
connected with the backbone not solely by muscles, but
also by joints or articulations with the large tians\erse
processes of the sacral vertebra ; it bears no bone coni-

FROG: EXTERNAL FEATURES AND BODY- WALL

47

it.

.parable with the clavicle, and there are in connection with
it no unpaired structures such as the sternum. The greater
part of each half consists of a long slender bone, the hip-
bone or ilium , corresponding in position to the scapular part
of the shoulder girdle, which runs downwards and back-
wards from the sacral vertebra, curving inwards on the
under side of the body to join its fellow. The junction is
enlarged into a flattened mass by the addition of several
elements which are more distinct while they are being
formed in development than
they are in the adult. Behind
lies a ridge_ of bone known as
the ischium , which consists at
first of two parts, one belonging
to each half of the girdle. A
slight groove marks the limits
-of this bone. In front a similar
ridge, not marked off from the
Ilium, is known as the pubis , and
represents a pair of pubic bones
found in certain other animals,
though its relation to the bones
so called in man is uncertain.

Ventrally, between the pubis and
the ischium, lies a triangular
piece of calcified cartilage, the
postpubic cartilage. In each of
the flat sides of the mass formed 23.— The hip girdle of a

by the union of these structures

is a round hollow, the leg-socket arc., Acetabulum ; il., ilium ; is.,

7 . i • -i r ischium; pb., pubic region of

Or acetabuluin , into which Ilts ilium \pb'., post-pubic cartilage

the head of the thigh-bone.

The upper arm contains a single bone, the upper arm
bone or humerus. This consists of a stout
shaft, swollen at each end, and bearing on its
inner side a ridge known as the deltoid ridge. The swelling
at the upper end is the head , and fits into the glenoid cavity
of the shoulder girdle. That at the lower end, the trochlea ,
is more irregular in shape and serves for the articulation of
the forearm bone or radio-ulna. In man, and in most
animals whose limbs are built upon the same plan as those

ac.

is.

48

MANUAL OF ELEMENTARY ZOOLOGY

of the frog, the forearm contains two bones, the radius and
the ulna , and traces of the fusion of these can clearly be
seen in the frog. The radius is the inner of the two com-
ponents of the bone, but its upper end lies partly in front of
the ulna.1 The upper end of the radio-ulna is hollowed to
receive the humerus at the elbow-joint, behind which it
projects as the elbow-bone or olecranon process. The wrist
consists of six small carpal bones arranged in two rows
across the limb. Those of the first row are named according
to their position radiate, intermedium , and ulnare .2 The

j_5) Distal carpals or tarsals ; 6, radiale or tibiale ; 7, intermedium ; 8,
ulnare or fibulare ; 9, centrale ; 10, radius or tibia ; ix, ulna or fibula :

12, humerus or femur ; 13, scapula or ilium ; 14, precoracoid or pubis ,

15, coracoid or ischium ; 16, coracoid fontanelle or obturator foramen ;

17’ glenoid cavity or acetabulum ; 18, clavicle; 19, metacarpais or
metatarsals; 20, phalanges; I.-V., digits.

second row contains in the early stages of its development
five bones, called distal carpals , corresponding to five digits,
but in the adult frog the third, fourth, and fifth of these
have fused.3 The palm contains five metacarpal bones.
The first digit is wanting, but the second and third have
each two bones and the fourth and fifth three, according

1 See p. 536.

1 For the names of the corresponding bones ip the rabbit and man,
see p. 536.

1 In many animals (but not in man) a bone known as the central
or os centrale lies between the two rows of bones of the wrist.

FROG : EXTERNAL FEATURES AND BODY-WALL

49

to the number of their joints. These bones are called
phalanges .

The bones of the leg correspond closely to those of the
arm. The thigh-bone or femur has a long, slender, slightly
curved shaft with a rounded head to fit into the acetabulum
and a wide condyle for articulation with the shank-bone ,
os cruris , or tibio-fibula. The latter, like the radio-ulna,
corresponds to two bones in man and many other animals^
showing traces of being formed by the fusion of an inner
or anterior shin-bone or tibia and an outer or posterior
fibula. The ankle, like the wrist, consists of two rows of
bones, which are here called tarsals. The first row contains
two bones, the tibiale, astragalus , or talus and the heel-bone ,
fibulare , or calcaneum. These bones are joined at each end
by a piece of cartilage. The second row consists of two
small distal tarsals. The metatarsus contains six meta-
tarsals, one minute and corresponding to a small extra toe,
the prehallux or calcar , which lies inside the first toe or
hallux, but does not project from the foot. The calcar has
one phalanx, the first two toes have each two, the third and
fifth toes three, and the fourth toe, which is the longest, has
four.

It will be seen that the fore- and hind-limbs and girdles
are built upon a common plan. The skeleton of this is
shown in Fig. 24. It may be traced in all animals which
are pentadactyle — that is, have fingers and toes. Neither of
the limbs of the frog conforms to it exactly.

The movements of the body and of its organs are brought
about by means of a tissue known as muscle.
This tissue is classed according to its function
as voluntary when it is under the direct control of the
will and involuntary when it is not under such control.
Between voluntary and involuntary muscle there is, gener-
ally speaking, a difference in fine structure : of this we
shall speak later (p. 117). Involuntary muscle usually
forms part of the wall of some internal organ such as the
stomach, bowel, bladder, or heart, and by its contraction
brings about changes in the width of this organ and thus
movement of the fluid it contains. Voluntary muscle is
usually found in the form of distinct organs or muscles ,
which are attached at their ends to two parts of the skeleton

4

50

MANUAL OF ELEMENTARY ZOOLOGY

and, by their contraction moving one of these lever- wise upon
the other, change the relative position ot the regions of the
body which they support. Sometimes the end of a muscle
may be attached by -a stout band or aponeurosis of con-
nective tissue to another muscle. A muscle has a belly
•of muscular tissue which is attached by tendons of a
peculiar kind of connective tissue. One of the two attach-
ments is called the origin , and this is made to a rela-
tively fixed part ; the other, called the insertion , is made
to a more movable part. Parts of the skeleton which aie

pIG 25. A diagram to illustrate the structure of “ perfect” joints.

at.c., Articular cartilage ; bn., bone ; lig., ligament ; wed., medulla or

marrow ; syu.c., synovial capsule.

thus movable upon one another must be provided with
joints. When the amount of movement which is possible
is small, the joint consists of an intervening layer of cartilage
or ligament, and is said to be imperfect. This kind of joint
is found, for instance, between the bones of the frog’s
shoulder girdle. When free movement is possible there is
a perfect joint. Here a convex surface of one structure
plays within a concave surface of another, the two
surfaces being separated by a fibrous bag, the synovial
„ capsule , which contains a watery fluid, the synovia , and
serves as a cushion. Outside the joint, ligaments hold the
movable pieces together. The muscular system of the frog

« FROG : EXTERNAL FEATURES AND BODY-WALL 51

us complicated, and we shall therefore only give an outline
of its general arrangement and mention a few of the more
important muscles.

The following table sets forth the general arrangement of
the muscular system :

A. Muscles of the Trunk.

1. Muscles of the lower side.

a. Muscles of the Belly.

e.g. Rectus abdominis , a wide band running along the
belly, divided lengthwise down the middle by the
connective tissue linea alba and transversely by
tendinous intersections.

Ob l i quits externus , a broad sheet at each side of the
body, arising from an aponeurosis known as the
dorsal fascia which covers the muscles of the back,
and inserted into the linea alba above the rectus
abdominis.

Obliquus interims and transversus , muscular sheets
within the external oblique.

By their contraction all these muscles lessen the size
ot the body cavity and compress the organs within it.

b. Rluscles of the Breast Region .

e.g. Pectoral is. large and fan-shaped, inserted into
the deltoid ridge of the humerus and consisting of a
sternal portion , arising from the pectoral girdle,
which draws down the arm, and an abdominal
portion , arising from the aponeurosis at the side
of the rectus abdominis, which draws the arm
backwards.

Coraco-radialis , arising from the coracoid and inserted
into the upper end of the radius. It bends the arm.

2. Muscles of the Back.

a. Muscle inserted into the Lower Jaw.

Depressor mandibulce , triangular, arising from the
suprascapula and inserted into the angle of the
lower jaw, which it draws downwards and back-
wards, thus opening the mouth.

b. Muscles inserted on the Fore-Limb.

e.g. Latissimus dorsi , triangular, arising from the
dorsal fascia and inserted into the deltoid ridge.
It draws back the arm.

Dorsalis scapula, in front of and similar to the
latissimus dorsi. It raises the arm.

c. Muscles inserted into the Shoulder Girdle.

e.g. Levator scapula, arising from the skull and
inserted into the under side of the suprascapula,
which it draws forward.

Serratus , arising from the little knobs which

MANUAL OF ELEMENTARY ZOOLOGY

represent ribs on the ends of the transverse pro-
cesses of the vertebrae. This muscle is inserted
into the under side of the suprascapula which it
draws backwards, outwards, or inwards according
to the division which is contracted.

d. Muscles inserted into the Hind-Limb.

e.g. Iliacus externus , arising from the ilium and in-
serted into the femur, which it rotates inwards.
Obturator internus , arising from the ischium and
pubis and inserted into the head of the femur,
which it rotates outwards.

#, Muscles inserted into the Hip Girdle.

e.g. Coc cy geo- iliacus, arising from the urostyle and
inserted into the ilium, which it holds firm as a
fulcrum for the movements of the hind-limb.

/. Muscles of the Backbone. .

t g. Longissimus dorsi , a band running the p o e
length of the back, divided by tendinous inter-
sections, which are attached to the transverse
processes, and inserted in front into the skull, it
straightens the back.

B, Muscles of the Head.

i. Muscles underneath the Head.

e. g. Sternohyoid from hyoid to pectoral girdle.

Geniohyoid from hyoid to chin.

Hyoglossus from hyoid to tongue.

Petrohyoid from hyoid to auditory capsule.

Mylohyoid , submandibular , or submaxillans , a sheet o
muscle running from side to side of the lower jaw.

These muscles alter the position of the floor of the

mouth.

%. Muscles of the Lower Jaw.

e.g. Temporalis and masseter , arising from the skull and in-
serted into the lower jaw, which they raise.

3. Muscles of the Eyeball.

Rectus superior , r. inferior , r. externus (or lateratis ), r.
internus (or medialis), arising from the skull in the
hinder part of the orbit and inserted into the eyeball.

Obliquus superior and 0. inferior , arising from the skull
in the front part of the orbit and inserted into the

eyeball. .

These muscles will be more fully described in the chapter

on the dogfish.

C. Muscles of the Fore-Limb.

I, Muscles for the Upper Arm. .

e.g. Deltoideus, arising from the scapula and episternum
and inserted into the humerus. It raises and pulls
forward the arm.

sm.

ad. Ins .

Fig. 26. — A ventral view of the muscular system of a frog.

ad. Ing., ad.ma ., Adductores longus and magnus ; anc., ancona3us ; cor. rad., cor-
aco-radialis ; cru., crureus ; dtd., deltoid ; e.ob., external oblique ; ex.cr., extensor
cruris brevis ; fl., flexors of hand ; gas., gastrocnemius ; gra., graciles ; l.a., linea
alba ; pct.ab., pct.st., abdominal and sternal parts of pectoral ; pns., pectinseus ;
r.ab., rectus abdominis ; s.mem., semimembranosus ; s.ten., semitendinosus ;
sar., sartorius ; sm., submaxillaris ; t.A., tendo Achillis ; t.i., tendinous inter-
sections ; tf., tibioflbula ; tib.ant., tib.post. tibiales anticus and posticus ; x.c.,
xiphoid cartilage.

B is a ventral view of the thigh after removal of certain muscles.

53

54

MANUAL OF ELEMENTARY ZOOLOGY

2. Muscles for the Fore- Arm. _ ^

Triceps brachii or anconceus , arising from the scapula and
humerus, and inserted into the upper end of the ulna. It
straightens the arm.

There is no Biceps muscle in the arm of the frog.

3. The muscles of the Wrist and Fingers are numerous and

complicated.

D. Muscles of the Hind-Limb.

I. Muscles of the Thigh. . .

e.g. Adductores magnus and longus, large muscles arising-
from the pubis and ischium, lying on the front ot the
thigh, and inserted into the femur near its lower end-
They draw the thigh towards the body.

Fig. 27.- — A dissection showing the
muscles which rotate the femur
of a frog. — After Ecker.

fe., Femur ; il., ilium ; il.ext., il;acus
externus ; obt.int., obturator intern us.

Jliacus internus , arising from the base of the ilium and in-
serted into the femur. It draws the thigh away from
the flank.

Fectineus, arising from the pubis and inserted into the
femur. It draws the thigh away from the flank.

Sartorius, a long, narrow band arising from the lower end
of the ilium, lying obliquely upon the adductor magnus,
and inserted into the tibia on its inner side near the
end. It bends the knee and helps to draw the thigh to
the flank.

FROG : EXTERNAL FEATURES AXE BOD Y-WALL

55

Gradies major and mi?ior, muscles arising from the
ischium, lying along the inner side of the adductor
magnus, and inserted into the inner side of the head of
the tibia. They bend the knee.

Sennmembranosus , a stout muscle arising from the
ischium, lying on the back of the thigh, and inserted'
into the back of the head of the tibia. It bends the knee.

Semitendinosus, a long, thin muscle arising by two heads
from the ischium and inserted into the proximal end of
the tibia. Its action is like that of the semimembran-
osus.

Triceps extensor cruris or t.femoris, a very large muscle
inserted into the front of the tibia just below the head
of the latter, but arising from the pelvic girdle as three-
separate muscles, the rectus femoris, the vastus lateralis
or glut ceus magnus , and the vastus medialis or crurcus _
All these lie on the front of the thigh, and their actioru
is to straighten the leg.

2. Muscles of the Shank.

e.g. Feroyiceus , a long muscle, which arises from the end of
the femur, lies along the side of the tibio-fibula, and is-
inserted into the end of the tibia and the calcaneum..
It straightens the leg.

Gastrocnemius , a large, spindle-shaped muscle which
forms the “ calf.” It arises from the hinder side of the
end of the femur and tapers into the long tendo Achillis-
(or t. calcaneus ), which passes under the ankle joint and
ends in the sole. It straightens the foot on the shank.

Tibialis anticus, arising from the front of the femur by a.
long tendon, lying in front of the shank, and dividing into-
two bellies, which are respectively inserted into the astra-
galus and calcaneus. It bends the foot on the shank.

3. The muscles of the Ankle and Toes are numerous and

complicated.

It should be noted that the muscles which move t; e
limbs are so arranged that to those which produce any
movement there are opposed others which reverse it.

Thus, in the hind limb :

The thigh is abducted (drawn away from the flank) by the iliacus
externus and pectineus, adducted by the adductores, graciles,
semimembranosus and semitendinosus, rotated inwards on its axis by
the iliacus externus and rotated outwards by the obturator intcrnus..

The shank is flexed by the sartorius, graciles, semimem-
branosus, and semitendinosus, and extended by the triceps femoris
and peronaeus.

The ankle is flexed by the tibialis anticus and peronaeus. and
extended by the gastrocnemius and extensores eruris.

56

MANUAL OF ELEMENTARY ZOOLOGY

In the fore limb : , 1 , . . , , . , , .1

The upper arm is abducted by the deltoid, adducted by the

abdominal part of the pectoral, drawn down by the latissimus dorsi
and the sternal part of the pectoral, and raised by the dorsalis

scapulae (with other muscles). , , ,

The forearm is flexed by the coraco-radiahs and extended b> the

anconaeus.

No muscle has precisely the same action as any other
and by the combined use of muscles very various and

complicated movements are executed. .

Muscles ordinarily contract because stimuli reach
them as impulses along nerves (p. 9 3b The irritabi ltv
which makes them responsive to these stimuli is also

shown by their contraction when artificial stimuli are
directly applied to them by pinching, heating, electric

shocks, etc. (see p. 738).

It would not be possible here to describe the various
modes of locomotion of the frog (p. 32), even ll
Locomotion. were at present thoroughly understood.

Something must, however, be said about crawling, m
which the frog practises what is the primary mode of loco-
motion of four-footed animals. In it the body is levered
forwards over the ground by the limbs, which are used in
diagonal pairs, the left hind-limb immediately after, or
practically with, the right fore-limb, and similarly the right
hind-limb with the left fore-limb. Fig. 28 shows a toad
moving in this way.

CHAPTER III

THE FROG : VISCERA AND VASCULAR SYSTEM

The food of the frog is received and digested by a winding
tube, known as the gut or alimentary canal ,
sysSm.tary which runs from mouth to cloacal opening and
has a muscular wall lined by a soft, glandular
mucous membrane (p. in). The gape of the mouth lies
between two jaws , of which the upper is not movable,
but the lower is hinged. There are no teeth in the lower
jaw, but the upper bears a row of maxillary teeth, and
a patch of vomerine teeth is found
on each side of the roof of the mouth.

The teeth are small, sharp-pointed
structures, consisting of a base and a
spike or crown. The greater part of
the crown is composed of ivory or
dentine , but the base is formed of
bone, and the crown is covered by a
cap of very hard substance known
as enamel , and both are pierced by a
core of soft tissue called the pulp
(Fig. 30). The teeth are all alike,
and all fused to the surface of the
bones that carry them. As they are

destroyed by use they fall out and are replaced one by one.
On the front part of the roof, beside the vomerine teeth,
open the internal nares (p. 102). The tongue is a muscular
structure arising from the front part of the floor ot the
mouth and forked at its free end, which is directed back-
wards when it is at rest. In taking food the tongue is
turned over and its free end thrown out of the mouth,
wiping up, as it goes, a sticky substance secreted by glands

57

Fig. 29. — Two of the
maxillary teeth of a
frog, seen from the
outside of the jaw.

b. , Base of the tooth; cr.,
crown ; m., edge of the
maxilla.

58

MANUAL OF ELEMENTARY ZOOLOGY

in the roof of the mouth so that the prey adhere to it.
Behind the angle of the jaw is a region known as the
pharynx , into which open, at the sides ol its root, the pair
of Eustachian tubes which lead to the drums of the ears,
and below, in the male, a pair of vocal sacs which are

inflated and act as reson-

.6.

ators during croaking. In
the middle of the floor ol
the pharynx is a slit-like
opening, the glottis , which
leads into the wind-pipe
(Figs. 31 and 537).

From the pharynx a tube
known as the gullet or
oesophagus leads backwards-
in the body cavity to the
maw or stomach , which is
spindle-shaped and separ-
ated by a slight constric-
tion, the pylorus , from the-
bowel or intestine. The first,
part of the intestine, known,
as the duodenum , is narrow
and turns forward so as to-
rn. lie parallel with the stom-
ach. It is succeeded by
another narrow tube, the
ileum , which runs back-
wards in several coils.
Duodenum and ileum are

Fig. 30.— A vertical section through t0o-ether known as the small
a tooth and part of the maxilla of intestim . at its hinder end

a !‘°s' this region opens suddenly

b.. Base of the tooth, composed of bone • . a much wider tube, the

(cement): d., dentine ; e.} enamel ; m., r ,1

maxilla; o.p.c., opening of the pulp yedum. the leilgtll Ot tile

cavity- small intestine is from 4 to*

3 inches ; that of the rectum is about an inch and a quarter.
The internal surface of the intestine is increased by folds ol
its lining. These are transverse in the duodenum and
longitudinal in the ileum, flhe rectum passes without
distinct anus into a cloaca , which receives ventrallv a thin

THE FROG : VISCERA AND VASCULAR SYSTEM 59

walled, bilobed sac, the urinary bladder , and dorsally the-
ducts of the kidneys and in the female those which bear
the eggs.

Besides numerous small glands in the mucous membrane,
the alimentary canal receives the secretions of two large
glands, the liver and the pancreas. The liver is a large,
reddish-brown structure in the forepart of the belly. It

pn.s. a.ch.p.

Fig. 31. — A longitudinal median section through the head of a frog,

aq ., Aquaeductus cerebri; a.ch.p anterior choroid plexus; br ., bronchus; c.c.
central canal of spinal cord ; cb., cerebellum; ccr.h., left cerebral hemisphere £
e.n., nostril; Eu., Eustachian tube;/.Af., foramen of Monro; gls., glottis"
i.n., internal narial opening ; in /., infundibulum; lar, larynx: lg., left lung;
m.ob., medulla oblongata; o.l., optic lobe; aes., oesophagus; olf.l., olfactory
lobe; p.ch.p ., posterior choroid plexus; pit., pituitary body; pn.s., pineal
stalk ; III. v., third ventricle ; IV.v., fourth ventricle.

consists of a right and a left lobe and a small median lobe
which unites them. The left lobe is the larger and is itself
deeply clelt into two. Between the right and left lobes lies
the gall-bladder , which receives the green bile secreted by
the liver and passes it by the bile-duct into the duodenum.
The pancreas is an oblong, creamy-white structure 1} ing
between the stomach and duodenum. It is traversed by
the bile-duct, into which it passes the pancreatic juice which
it secretes.

The food is not chewed, but is swallowed whole, the

a. muse. <*'

i V jijs /

/ MW

y/f u

spit C- y jte t

\\ /Jm

a.ab.

pIG> -^2. — A male frog dissected from the ventral side.

v Anterior abdominal vein, cut short, ligatured, and turned back ; a muse
cut edge of abdominal muscles ; bl., urinary bladder ; c d., common duct of
gall-bladder and pancreas; d.ao., dorsal aorta ; du., duodenum; f.b fat
body; fem.v., femoral vein; g.b., gall-bladder; hi, heart ; hy.n hypoglossal
nerve ; im., ileum ; i.v.c., inferior vena cava ; k kidney; k.d, kidney duct
with vesicula seminalis ; lr., liver ; o., point at which c.d. enters the duodenum ;
pcs., pancreas ; pl.v., pelvic vein ; r.L, right lung ; rm., rectum , r-p-v., renal
portal vein ; sar., sartorius muscle ; sm., mylohyoid muscle , sp., spleen , st.,
stomach ; t., testis v.v., vesical vein.

60

THE FROG : VISCERA AND VASCULAR SYSTEM 61

only use ol the teeth being to prevent the escape of the

Di estion Pre3b which they can the better do because they
slant backwards. In the stomach the food
meets the gastric juice , secreted by the glands of the
mucous membrane of the stomach (p. in). The important,
contents of this watery juice are free hydrochloric acid and
the organic substance pepsin , which starts the digestion of
protein, turning it into a more soluble form (peptone).
Mixed with the juice, the food is churned by rhythmical
contraction of muscle-layers in the stomach wall, and
thus is killed, disinfected of bacteria by the acid, softened,
partly dissolved by the action of the pepsin, and broken
up. Pepsin is one of a very important class of substances,
found in living bodies, which are organic catalysts,
having the power of bringing about changes in other
substances without themselves undergoing change, and
of doing this even though they be present in very small
quantities in a large mass of the substance acted upon.
These agents are called ferments or enzymes.

Enzymes act not only in digestion but also in many chemical events
Enzymes i*1 ce^s °f the body. Oxidation, the preparation of

excreta, and other processes take place with their aid.
The classes of enzymes are known by names derived from those of the
substances upon which they act, with the termination -ase. Thus
pepsin is a protease. The protease of pancreatic juice (see below), is
trypsin , the carbohydrase is amylase , the fat-splitting enzyme is lipase.

From time to time a ring of muscle known as th e pyloric
sphincter , which guards the opening of the duodenum,
relaxes and lets through partly digested food into the
intestine, where it meets three alkaline juices, the bile,
the pancreatic juice , and a juice known as the succus
enter icus , which is secreted by the intestinal wall. Of
these the pancreatic juice is the most powerful, turning
all three classes of organic food-stuffs, each by means ot a
distinct enzyme, into the soluble substances mentioned
on pp. 9, io. The action of the bile and succus entericus-
is subsidiary to that of the pancreatic juice, and the bile
is also partly an excretion. The food thus rendered
diffusible is absorbed by the activity of the intestinal
lining. The movement of the food along the alimentary
canal is brought about by the working of a muscular

<02

MANUAL OF ELEMENTARY ZOOLOGY

Functions of
the Liver.

layer in the wall of the latter (see Fig. 58), down which
waves of contraction pass, pressing the contents before
them. This process is called peristalsis . Finally the
undigested portion of the food is driven out through the

rectum and cloaca to the exterior.
The secretion of bile is not the
only function of the liver.
That organ is the great
chemical workshop and
storehouse of the body. In it a
part of the excess of carbohydrate
and fatty food taken during the
summer is stored for use during the
winter sleep and the breeding
season. The fat is stored in droplets,
the carbohydrate in the form of
glycogen or animal starch, which,
when it is to be transferred to other
parts of the body, is converted into
sugar and cast into the blood. In
the liver also the ammonia which
results from the decomposition of
proteins is converted into urea ready
frog’s eggs. The one for excretion by the kidneys, and

various other chemical changes
take place.

We must here mention the organs
known as the ductless

1 in

FlG jo- —Two individuals
of the same age from
the same batch of

on the right has
changed normally
into a frog ; that on
the left had the rudi-
ment of its thyroid
removed and has not
become adult but has
grown into a giant
tadpole. — From Hal-
dane and Huxley.

Ductless

Glands.

glands or endocrine or-

but into the blood
internal secretion.

gans , which, while they
manufacture substances of im-
portance to the body, discharge
these products not through a duct
or lymph by the process known as
Substances are, of course, continually
being passed into the blood by every organ, but among
these it is important to distinguish between (a) the waste
products of metabolism, (b) substances which are used in
bulk, such as the sugar discharged by the liver, and (c) the
u chemical messengers ” or hormones (p. 21), ot which min-
dte quantities are secreted as a stimulant or inhibiting agent

THE FROG: VISCERA AND VASCULAR SYSTEM 63

to the action of other organs. It is to this latter class that
the characteristic products of the ductless glands belong.

The thyroid glands of the frog are a pair of small, rounded, pinkish
bodies lying on the external jugular veins. Their secretion, which
contains an organic compound of iodine {thyroxin), has an important,
but not well understood, action in maintaining the normal working
of various parts of the body. The change from tadpole to adult is

Fig. 34. — Two frogs nineteen days after operation. From that on
the left only the anterior lobe of the pituitary body has been
removed ; from that on the right both anterior and posterior
lobes. Lack of the hormone from the posterior lobe has caused
in the right-hand frog pallor due to non-expansion ot the pigment
in the pigment cells. — From Hogben.

brought on by it. In man, the thyroid is a single, median body (as it
Is in the embryo of the frog) ; and defect of its secretion has very
serious consequences both for the young and for adults, in whom
the lack of it produces both mental dullness and physical disorder
.(see p. 673).

The adrenal bodies (so-called suprarenal glands) aie small yellowish
masses lying on the ventral surface of the kidneys. I hey consist of
two kinds of tissue, which in the frog are mixed, while in man one,
the cortex , is a layer around the other, the medulla. The secretion
of the cortex is essential to the maintenance ot life, but its mode of

64

MANUAL OF ELEMENTARY ZOOLOGY

action is unknown. The medulla secretes a substance {adrenalin)
whose effects resemble those of the activity of the sympathetic nervous
system (p. 92), influencing, positively or negatively according to the
organ affected and its condition, the “ tone ” of muscle and the activity
of glands. At times of excitement and exertion its discharge tunes up
the body for action (see p. 22).

The thymus is a small body which lies behind and above the angle
of the jaw on each side. Its functions are unknown.

The pituitary body lies in the skull below the brain (see p. 87).
It is a gland of many hormones. It consists of an anterior and a
posterior lobe, of different function. The anterior lobe arises solely
from an ingrowth, known as the hypophysis , from the roof of the
mouth. Its secretion acts not only directly, by means of hormones
which stimulate growth, particularly that of bone, and sexual pro-
cesses, but also indirectly by hormones which stimulate the thyroid
and the cortex of the adrenal body. The posterior lobe is partly
hypophysial, partly a ventral appendage of the brain (the infundi-
bulum). Its secretion is the means of dilating the pigment cells in
the frog’s skin (p. 35), increases blood pressure by contracting vessels,
and has various other effects.

Hormones are also secreted by ordinary glands. Thus the pro-
duction at the right moment of large quantities of gastric and pan-
creatic juice is brought about by the stimulation of their glands,
partly by impulses through the nervous system during feeding, and
partly by hormones, known as secretin s, formed in the wall of the
alimentary canal under the influence of substances in the food and
carried by the blood to the glands. Lnsulin , whose defect causes
diabetes, is produced by certain cells ( islets of Langerhans) in the pan-
creas. The testes and ovaries not only form spermatozoa and ova but
also liberate hormones, of which some influence the development of
secondary sexual characters in the growing individual, and others
bring about events in the process of reproduction.

The spleen is a small, round, dark red body, lying in the
mesentery opposite to the beginning of the rectum. Its
cells remove and destroy effete “ red corpuscles ” (cells,
see p. 122) of the blood, and certain minute parasites. It
is also concerned in the preparation of nitrogenous waste
matter for excretion. In some animals at least, including
man, it acts as a reservoir of blood. Its removal is not fatal.

The fat bodies are two orange-coloured tufts of flattened
processes, attached in front ot the ovary or testis to the
dorsal wall of the body cavity. They consist of fatty tissue
(see p. 120) which, like the reserves in the liver, increases
during the summer and decreases during the winter sleep,
when it is being drawn upon for nourishment, particularly
in the preparation of germ cells for breeding in the spring.

THE FROG: VISCERA AND VASCULAR SYSTEM 65

La.

The heart of the frog is a hollow, conical, muscular organ,
which lies, with the apex backwards, in the body
System^ Heart cavity, between the breast-bone and the gullet.

It is enclosed in a thin sac, the pericardium ,
whose cavity is a part of the body cavity (coelom) separated
trom the rest during development, the heart having the
same relation to it that the alimentary canal has to the
pleuroperitoneal or
general body cavity — -
that is, being covered
by a continuation of
the pericardial mem-
brane as the gut is by
the peritoneum. The
heart contains five
chambers. Of these
the most conspicuous
is the ventricle , a
large, conical struc-
ture, with thick, mus-
cular walls, from
which arises in front,
on the right side of the
ventral surface, the
much smaller, tubular
truncus . arteriosus.

The right and left
auricles or atria are
relatively thin walled
chambers, the right
larger than the left,
separated by a septum
and lying in front of

the ventricle, into which each opens. On the dorsal surface
of the heart, opening into the right auricle, lies the still
thinner walled, triangular sinus venosus ; into the left auricle
opens the pulmonary vein (p. 71).

The openings between these chambers are guarded by
certain valves or folds of the lining ot the heart. Two
simple lips of the opening between the sinus and right
auricle are the sinu-auricular valves ; these allow blood to

Fig. 35 — The heart of a frog, seen from
the ventral side.

c.a., Carotid arch; c.gl., carotid gland; i.c.,
internal carotid artery ; l.a., I'ngual artery ;
pc.a., pulmocutaneous arch; p.m., peri-
cardium; r.au., /.aw., right and left
auricles; s.v.c., superior vena cava; sy.a.,
systemic arch; tr.a., truncus arteriosus;
v., ventricle.

66

MANUAL OF ELEMENTARY ZOOLOGY

tZ

p.v.

flow into the auricle, but when it tends to flow the other
way fold over and meet to oppose it. The edge of the
auricular septum is cleft and projects into the ventricle
as two flaps, the auriculo-ventricular valves . Each of these
is connected with the walls of the ventricle by fine cords,
the chordae tendinece , and thus, while blood can pass from
auricles to ventricle, its reflux is prevented by its raising
the valves, which are kept from turning back into the

auricle by the chordae tendineae. The opening from

ventricle to truncus
is guarded by three
semilunar valves ,
shaped like watch-
pockets, which are
spread out by any
reflux of blood, so
that by meeting one
another they stop it.
The truncus arteri-
osus is divided inter-
nally by a second
ring of semilunar
valves into two un-
equal parts, a long
Fig. 36. —The heart of a frog, removed COnus arteriosus or
from the pericardium and seen from the f,ylanaiUm .next the
back with the sinus venosus opened. wntriclej and a short

iv.c., Inferior vena cava ; p-v., pulmonary veins,
r.au., l.au., right and left auricles;, s.au.,
opening from sinus to right auricle ; s.7>.c.,
superior vena cava ; s.v,, sinus venosus , tr ..

S.7J.

r.au .

s.v.c .

s.au.

i.v.c .

bupcuoi vena -

branches of truncus cut across ; v.y ventricle.

ventral aorta or syn-
angium . The conus is
incompletely divided
longitudinally by a
spiral valve into a cavum aorticum- , which begins dorsal
and curves round by the right to become ventral, and
a cavum pulmocutaneum, which begins ventral and curves
round by the left to become dorsal. The synangium is
completely divided into a dorsal and a ventral chamber.1

1 The septum which makes this division ends towards the heait by
cutting across the hollow of one ot the second row ot semilunar valves.
It is from the outer side of this valve that the spiral valve starts.
Thus it comes about that the outer ends ot the cavum aorticum an
cavum pulmocutaneum are each guarded by one and a halt valves.

THE FROG : VISCERA AND VASCULAR SYSTEM 67

The dorsal chamber communicates behind with the cavum
pulmoeutaneum and in front with the blood vessel to the lungs

Fig. 37. — A ventral view of the heart of a frog, opened to
sriow the internal structure. The ventral wall of the
truncus, ventricle, and auricles has been removed, with
part of the spiral valve.

■au.z’., Auriculo-ventricular valves ; c.a ., carotid arch ; c.ao., cavum aorticum ;
cpu., cavum pulmoeutaneum ; ch.i., chordae tendineas ; l.au., left auricle ;
o.p.v., opening of pulmonary vein ; o.pc., opening of dorsal division of
synangium, by which blood passes from the cavum pulmoeutaneum to the
pul mocu taneous arch ; pc. a., pulmocutaneous arch ; r.au., right auricle ;
s.au., sinu-auricular opening with valves ; s/., first row of semilunar
valves; si' . , semilunar valves of second row ; sP. i, the semilunar valve
from which the spiral valve starts ; iA.2, small semilunar valve at
end of cavum pulmoeutaneum ; sP.3, a small part of a large semi-
lunar valve, of which the rest extends across that portion of the
front wall of the truncus which has been removed ; sp.v ., spiral
valve; sy.a., systemic arch; tr.a., wall of truncus arteriosus; tr
one of the two bundles of arteries into which the truncus divides;
v., ventricle.

(pulmocutaneous arch) ; the ventral chamber communicates
behind with the cavum aorticum and in front with the
blood vessels known as the systemic and carotid arches.

6S

MANUAL OF ELEMENTAL Y ZOOLOGY

The function of the heart is, by the contractions of its
muscular wall which are known as its beat , to
Heart-beat c}rive blood through the vascular system to

all parts of the body. The contraction starts in the sinus
venosus, driving the contained blood into the right auricle.
Meanwhile the left auricle is filling by the inflow of blood
from the lungs through the pulmonary vein. The auricles
now contract simultaneously, driving the blood into the
ventricle. The sinus is beginning to relax, but the reflux
of blood into it is prevented by the sinu- auricular valves.
The right-hand side of the ventricle receives the blood
from the right auricle and the left-hand side that Irom the

left auricle, and these por-
tions of blood mix slowly
because a great part of the
hollow of the ventricle is
spongy, owing to the pres-
ence of muscular projections
known as columnce carnecu .
The ventricle contracts im-
mediately after the auricles,
the auriculo - ventricular
valves preventing the pass-
age of blood back into the
latter. The effect of the
contraction of the ventricle
is therefore to drive the
blood onward into the
truncus arteriosus. Since this is on the right side of
the ventricle, it will receive first the blood irom the
right auricle. Both cavum aorticum and cavum pulmo-
cutaneum are filled, but since the pressure in the
carotid and systemic arches is higher than that in the
pulmocutaneous arch, blood is first driven into the latter.
As the ventricle continues to contract, the pressure ot the
blood rises until it is high enough to overcome the resistance
in the svstemic and finally in the carotid arches. At. the
same time the contraction of the truncus arteriosus brings
the spiral valve into a position in which it shuts oft the
cavum pulmocutaneum. Thus the blood from the left
auricle (and therefore from the lungs), which is the last to

a(l. vnl.

FlG. 38. —Capillaries in the web
of a frog’s foot.— After Allen
Thomson.

at L, Arteriole; cap., capillaries;
vnl., venule.

THE FROG: VISCERA AND VASCULAR SYSTEM 69

Circulation ot
the Blood.

enter the truncus, passes along the cavum aorticum into
the systemic and carotid arches. The blood in the
systemic arch is a mixture of that from the right and left
auricles ; the final portion which passes into the carotid
comes only from the left auricle. The meaning of this
separation of the blood will be seen later.

To and from the heart leads a complicated system of
blood vessels , through which the red blood is
driven by the heart-beat. The vessels which
lead from the heart are called arteries ; those
which lead to the heart are veins. The arteries have
thick, muscular walls, and after many subdivisions become
small vessels known as arterioles.

These in turn divide into minute, very
thin-walled vessels called capillaries ,

that lie among the tissues in the form

of a meshwork, which in active tissues, c sp -<—h
such as glands and muscles, is ex-
ceedingly fine, so that the blood is
brought close to every part. From
the capillaries the blood is collected
into small venules which join to form
the veins. The walls of the veins are
thinner and less muscular but more
elastic than those of the arteries, and
many of them contain small watch-
pocket valves, placed with the opening
of the pockets towards the heart so as
to prevent the blood from being driven in the wrong direction
when the vessels are compressed by the movements ol the
body. Through heart, arteries, capillaries, and veins there
takes place a circulation of the blood , which can be seen
under the microscope in the capillaries of the thin web
between the toes of the frog’s foot. In the arteries the blood
flows fast and with jerks, which are caused by the beating ot
the heart and known as the pulse. In the arterioles the in-
creased friction owing to the increased surface ot the numer-
ous branches obliterates the pulse. In the capillaries the
increased area lessens the rate of flow. In the veins the
blood is flowing back to the heart evenly and less last than
in the arteries, though faster than in the capillaries.

Wir'l

Fig. 39. — Diagrams of
a valve in a vein.

A, Part of the wall of the
vein from within ; B,
longitudinal section of
the vein, showing the
position of the valve when
blood flows from the
direction of the capillaries;
(c) towards the heart (h) ;
C, similar section showing
how reflux is prevented
by the valve.

7o

MANUAL OF ELEMENTARY ZOOLOGY

The supply of blood which an organ receives depends on
two factors : (i) the width of the small blood
thVc!rcu°ia-°f vessels in the organ, (2) the pressure under
tion- which the blood is flowing. When an organ

such as a muscle is active, its small vessels are caused to
dilate by the presence ol carbonic acid (p. 21) and other
products of the activity ot its tissues, and also by the action
of the nervous system. Now any dilatation ot blood
vessels, by enlarging the bed of the blood stream, tends to
lower the general blood pressure, and thus both to diminish
the local effect of enlarging the vessels, and also to have
injurious results in other organs. These tendencies,
however, if they be on a sufficient scale, are counteracted
through the nervous system in two ways — by an accelera-
tion of the rate of the heart-beat, and by the contraction
of vessels in other parts of the body (in this case, ot the
alimentary canal and of the spleen), so that the total
capacity of the vascular system is not increased.

From the truncus arteriosus there arise on each side three
arteries, which are for some distance bound
8iood vessels. t0getper5 so that they seem to be a single

vessel. The hindermost of these is the pulmo cutaneous
arch , the middle the systemic arch , the foremost the
carotid arch or common carotid artery. After separating,
the three arches continue to run outwards, diverging as
they go. The pulmocutaneous arch divides into the
pulmonary artery for the lung and the cutaneous artery for
the skin and mouth. The carotid arch gives a lingual or
external carotid artery to the muscles of the tongue and
hyoid, and then becomes the internal carotid artery which
bears a round swelling due to the fact that it here breaks
up into a number of small vessels which reunite. This
swelling is the carotid labyrinth , often inappropriately
called the carotid gland. 1 he friction of the blood against
the large surface provided by its numerous small vessels
is the cause of the high pressure in the carotid arch.
Beyond the carotid gland the artery runs forwards and
upwards towards the head, where, after an orbital
( stapedial ) branch to the orbit and roof of the mouth, it
passes into the skull and supplies the brain. The systemic
arch curves upwards and backwards round the oesophagus

THE FROG : VISCERA AND VASCULAR SYSTEM 71

e.c.

to join its fellow in the middle line below the backbone.
On its way it gives off
an oesophageal artery
to the oesophagus, an
occipito-vertebral artery
to the head and back-
bone, and a large sub-
clavian artery to the
arm. Just before join-
ing its fellow, the left
systemic arch gives oft
backwards the large
coeliaco-mesenteric ar-
tery. This divides into
an anterior mesenteric ,
to bowel and spleen,
and a coeliac , which
supplies the stomach
after giving a hepatic
branch to the liver. The
vessel formed by the
junction of the systemic
arches is the dorsal
aorta. It runs back-
wards immediately be-
low the backbone,
giving off paired renal
arteries to the kidneys,
ovarian or spermatic
arteries to the gen-
erative organs, and a
small median posterior
mesenteric artery to the
rectum, after which it
divides into two iliac
arteries to the legs and
abdominal muscles.

The blood from the
lungs is returned by
the right and left
pulmonary veins , through a short common pulmonary

Fig. 40. — A diagram of the arterial
system of the frog, seen from the
ventral side.

The lungs (lg.), kidneys ( k .), and right testis
(t.), are shown. The course of the
venous blood is shaded.

Arteries : a.mes., anterior mesenteric ; c.a.,
carotid arch ; c.l., carotid labyrinth ;

c.m., coeliaco-mesenteric ; ccel., cceliac ;
cu., cutaneous ; d.ao., dorsal aorta ;
e.c., external carotid ; f.b., to fat body ;
g., gluteal ; h., hsemorrhoidal ; hep.,

hepatic ; i.c., internal carotid ; il.,

common iliac ; p., pulmonary ; p.m.a.,
posterior mesenteric ; pc.a., pul-
mocutaneous arch ; r., renals ; scl.,

subclavian ; spm., spermatic ; sy.a.,
systemic arch ; v., occipitto-vertebral.

72

MANUAL OF ELEMENTARY ZOOLOGY

vein to the left auricle. From the rest of the body the blood
is returned to the sinus venosus by three large veins, the

right and left sup-
erior venae cavce or
precaval veins in
front, and the
median inferior
vena cava or post-
caval vein behind.

1

precaval is

Fig. 41. — A diagram of the venous system
of the frog.

u.ab.y Anterior abdominal vein ; br., brachial ; c.v.,
cardiac (conducts backward from wall oi heart).
cut., cutaneous; d.L, dorsolumbar; e.j. , external
jugular; /., femoral; h.p., hepatic portal; hep.,
hepatic ; i.j. , internal juguiar ; i.v.c ., inferior vena
cava; inn ., innominate; /., lingual; md ., mandi-
bular; pi. , pelvic; pul., pulmonary; r., renals ;

r. p., renal portal ; sc. , sciatic ; s.scp., subscapular ;

s. v., sinus venosus (seen through ventricle) ; s.v.c.,
superior vena cava; scL, subclavian ; sp/n., right
spermatic ; x., vessel joining sciatic to femoral.

Each
formed by the
junction of three
veins, the external
jugular , innom-
inate, and sub-
clavian. The
external jugular
is fed by a lingual
vein from the floor
of the mouth and
a mandibular
from the lower
jaw. The innom-
inate arises by the
junction of an
internal jugular
from the head and
a subscapular
from the shoulder
and back of the
arm. The sub-
clavian receives
the brachial from
the arm and the
great musculo-
cutaneous from
the skin, the muc-
ous membrane of

the mouth, and
many head and trunk muscles. The inferior vena
cava arises by the junction of several renal veins from

THE FROG : VISCERA AND VASCULAR SYSTEM 73

the kidneys and ovarian or spermatic veins from the
generative organs, and, just before it enters the sinus,
is joined by two hepatic veins from the liver. Blood is
returned from the legs by a femoral vein on the outside
and a sciatic on the inside of each limb. Each femoral
vein divides on reaching the trunk into a renal portal
and a pelvic . The former receives the sciatic and runs
to the kidney, just before entering which it receives the
dorsolumbar vein from some of the muscles of the back.
In the kidney the vein breaks up into capillaries, which are
collected, with those of the renal artery, to give rise to the
renal veins. Thus it comes about that much of the blood
in the renal veins has passed through two sets of capillaries,
■one in the leg and another in the kidney. Such an arrange-
ment, in which the blood having passed through one set of
capillaries is then sent through a second, is called a portal
system. The pelvic veins of the two sides lie in the abdominal
wall and join to form the anterior abdominal vein which runs
forwards above the linea alba in the middle of the belly (see
p. 51). This vessel receives a small vesical vein from the
bladder, several pairs of vessels from the recti muscles of the
abdomen, and a little backward vessel from the heart-wall.
It ends in front by passing into the liver and there breaking
up into capillaries again. The blood from the stomach,
bowel, pancreas, and spleen is gathered up into a great
hepatic portal vein , which also breaks up in the liver. Thus
the liver has a portal system, which is ted with blood
(a) from the dorsal aorta, ( b ) from the anterior abdominal
vein, (c) from the hepatic portal vein, and discharges by
the hepatic veins into the inferior vena cava.

The general course of the circulation in the trog is
summed up in the table on p. 74. The thick
lines indicate “ venous ” blood the narrow,
lines “ arterial blood (see p. 76).

The elaborate arrangements whereby the blood circulates
through all parts of the body point to the tact
that it is a universal means ot transport between
them. It conveys nourishment trom the gut
to the rest of the body, oxygen and carbon dioxide between
the organs of respiration and the tissues (p. 76), the waste
products of metabolism from the tissues to the organs ot

Course of
the Circulation.

Function of
the Blood.

74

MANUAL OF ELEMENTARY ZOOLOGY

excretion, and various substances secreted into it by the
liver and’ other organs to the regions of the body where
they are made use of. It distributes hormones (p. 62).
It also conveys heat— which, inside as outside the body,
is set free by most chemical changes— from the organs
where there is much chemical activity, such as the muscles
and glands, to those where there is little, such as the

Sinus venosus
Right auricle
Ventricle

Truncus arteriosus

i

Lungs

i

Left auricle
Ven tncle

Truncus arteriosus

Fig. 42. — A diagram of the circulation of the blood in the frog.
Thick lines indicate venous blood, narrow lines arterial.

skeleton and nervous system, and to the surface ol the
body, where what is excessive is lost. In some animals,
as in the rabbit (p. 557) and man, so much heat is produced
that the body is kept constantly at a temperature which is
a good deal higher than that of its normal surroundings,
This does not happen in the frog, whose temperature is
only a few tenths of a degree above that of the air or water,
and varies with it. The frog is therefore said to be cold-
blooded.” The blood is also an extremely important

THE FROG : VISCERA AND VASCULAR S YSTEM 75

defence against the attacks of bacteria and other micro-
organisms. In a later chapter (p. 123) we shall consider
the warfare which its cells wage against these enemies.

The preceding paragraph must not be taken to indicate
that the blood comes itself into contact with
Lymph. the tissues. The blood vessels are completely

closed, and the tissues are actually bathed by another
fluid, known as lymph , which is produced by exudation
through the capillary walls. This fluid is gathered by small
lymphatic vessels into the big lymph sacs already mentioned,
whence it is pumped back into the veins by two pairs of
small contractile sacs known as lymph hearts. One pair
of these lies below the scapulae and opens into the sub-
scapular veins ; the other lies at the end of the urostyle and
opens into the femoral veins. The fluids in the coelomic
(pleuroperitoneal and pericardial) cavities are lymph.
The pleuroperitoneal cavity communicates with the
dorsal lymph sacs by minute openings ( stomata ) in the
membrane which separates it from them.

The respiratory organs of the frog are the lungs, the
shin, and the mucous membrane of the mouth.

Respiration The iungs communicate with the pharynx by
way of the glottis, which leads into a short,
wide windpipe consisting of the larynx or voice organ only,
without the long trachea, or windpipe proper, which is
found in animals with necks. The walls of this cavity are
supported by a pair of flat arytcenoid cartilages and a very
irregular ring, the cricoid cartilage. The lining of the
larynx is thrown into a pair of folds, the vocal cords.
Between these is a narrow slit, the rim a glottidis, through
which the air must pass to and from the lungs. The
cartilages are supplied with muscles, by which they can be
moved, so as to tighten the vocal chords and bring them
close together. In this condition the chords vibrate when
air from the lungs is forced between them, and produce a
sound which is the croaking of the frog. From the hinder
part of the windpipe an opening leads on each side to a short
tube known as the bronchus , which begins at once to expand
into the lung. The latter is a wide, thin-walled, elastic,
highly vascular sac, whose internal surface is increased by
numerous folds. The lungs of the frog are not enclosed,

76

MANUAL OF ELEMENTARY ZOOLOGY

like those of man, in a “ chest ” shut off by a midriff/but lie
free in the fore-part of the common body cavity, and the
mode of breathing (air renewal) is correspondingly different
in the two cases. In man, as in the rabbit (p. 547), it
consists in an enlargement of the chest, which draws air
into it, followed by a collapse which drives it out. In the
frog, air is pumped into the lungs by the mouth.

Expiration is by collapse of the elastic lungs and pressure from
abdominal muscles. In inspiration the mouth floor is raised by the
mylohyoid and jaw muscles, jaws and nostrils being closed and glottis
open. Strangely, the air so forced in is not pure but a mixture of that
just expired into the mouth and fresh air previously “ aspired ” through
the nostrils by lowering the mouth floor by muscles of the hyoid.
Aspiration, expiration, inspiration, and discharge of air through the
nostrils occur in that order. The jaws are closed throughout.

In the lungs an exchange of oxygen for carbon dioxide
takes place through the thin walls between
Arterial and tpe ajr ancj the blood in the lung capillaries.

1 he same process goes on in the very vascular
mucous membrane of the mouth and in the skin. Indeed,
when the frog is at rest respiration is performed more
through these organs than through the lungs. In the
tissues of the body the blood undergoes a reverse change,
parting with its oxygen to the protoplasm and receiving
from it carbon dioxide formed there. The blood which
has thus become poor in oxygen and rich in carbon dioxide
returns to the heart through the veins. Such blood is
therefore known as venous blood. It is oi a dark red colour.
On reaching the heart, this blood is directed, as we have
seen, principally to the lungs, skin, and mucous membrane
of the mouth, there to be oxygenated again. The blood
from the skin and mouth mingles on its way back to the
heart with the venous blood, but that from the lungs returns
separately to the heart and is then sent iorth again to the
tissues through the arteries. Oxygenated blood is therefore
called arterial. It is of a bright red colour. It will be
noted that the pulmonary artery contains venous blood, the
pulmonary vein arterial blood. It will also be seen that
the course of the circulation contains two circuits, one
short, passing through the lungs, and the other long,
passing through the rest ot the body, the blood re-
turning to the heart between the two. This is shown

THE FROG : VISCERA AND VASCULAR SYSTEM 77

on the diagram on p. 74. The two circuits are known
respectively as the lesser or pulmonary and the greater
or systemic circulations.

i.v.c. (ps.

a.ab.v.

Fig. 43. — The urinary and generative organs of a male hog.

a.ab.v ., Anterior abdominal vein, cut short and turned back; b/., urinary
bladder; cl. , cloaca; d.ao., dorsal aorta \/.b., base of fat body \/.b.v.,
vein of fat body ♦ femoral vein ; il.a., iliac artery \ . i.v.c. , inferior

vena cava ; k., kidney ; k.d., kidney duct ; tnso ., mesorchium , as., oeso-
phagus ; pl.v., pelvic vein; r.p.v., renal portal vein; r.v., renal veins ,
sc.v., sciatic vein; sr.b., adrenal body; t.v., spermatic vein; v.eff., vasa
efferentia; ves.setn., vesicula seminalis. The testes are not labelled, nor
are the sciatic plexuses, a portion of which may be seen beside the iliac
arteries.

A diagram of these organs will be found on p. 82.

78

MANUAL OF ELEMENTARY ZOOLOGY

The respiratory organs are engaged, as we have seen,
in ridding the body of carbon dioxide, and some
Excretonr water is also lost in the form of vapour through
these organs. A further loss of water in a liquid
form and the excretion of solids dissolved in it takes place
through the kidneys. These are a pair ol flattened, oblong,

pIG 44.— A diagram of a kidney of the frog, to show the arrangement
ot the tubules and blood vessels. One uriniferous tubule and a
portion of the vascular meshwork are shown separately. In
reality the blood vessels entangle the tubules.

cap.,

Capillaries; col.t., collecting tubule; glom., glomerulus; M.c Malpighian
capsule ; pe., peritoneum ; r.a., renal artery ; r.p.v., branch of the renal portal
vein • r.v., branch of a renal vein ; ur.t., uriniferous tubule (somewhat
unravelled ; note that its regions differ) ; ur.t'., places where other uriniferous
tubules open into the collecting duct ; W.d., Wolffian or kidnev duct.

dark red bodies which lie one on each side in the dorsal
lymph sac above the coelom and below the backbone.
Each consists of a mass of twisted uriniferous tubules , held
together by connective tissue and richly supplied with
blood vessels. Each tubule begins blindly m the substance
of the kidney as a thin-walled Malpighian capsule , whose
side is indented by a cluster of blood vessels, the glomerulus,
the rest of the tubule being glandular. The glomeruli
receive blood only from the renal artery, the tubules also
from the renal portal vein. The tubules open into collecting
tubes , which run across the kidney to enter the main duct of

THE FROG : VISCERA AND VASCULAR SYSTEM 79

the organ or Wolffian duct} This lies along the outer edge
of the organ and passes backward to open into the dorsal
side of the cloaca. A watery fluid, containing some of the
solids of the blood, but not the proteins, passes, under the
blood pressure, through the thin wall of the glomerulus
into the capsule and so down the tubules. In these it is
deprived by reabsorption of some of its substances (sugars
and certain salts) which would be a loss to the body and
receives certain others, notably urea. It is then the urine,
and is held in the bladder and voided at intervals.

Water

Regulation.

In animals which are completely terrestrial, as in the rabbit and in
man, the kidney tubules have a function which is little if at all
exercised in the frog — that of reabsorbing some of the water lost in
the glomerulus and so conserving for the body this
liquid — which is immensely important, on land is in-
evitably lost more or less rapidly by evaporation, and
can there only be replaced at the cost of a considerable expenditure
of energy. In respect of the maintenance of the correct proportion
of water in its body the frog, whose lite is spent in or near water, is
a fresh-water animal. For such animals there is danger not of
defect but of excess of water in the body, because, the concentration
of salts in the water they inhabit being less than that in their bodies,
water tends to enter them by osmosis. That would cause swelling 1 2 and
harmful dilution ot the blood and other body fluids. This danger is
met partly by excreting water, as in the urine, and partly by hindering
its entry — in some animals by a thick cuticle over most of the surface
and in all by the protoplasm ot the surface tissues, which has the
power of regulating, within limits, the passage of substances through
itself. When it is on land the frog is losing water by evaporation from
its skin, which, unlike that of a truly terrestrial animal, lets water
through fairly readily ; this loss is made good by absorption through
the skin when the animal is again in water. The absorption is regulated
by the action of the tissues of the skin ; how that is done is not known,
but it has been found to be influenced by the nervous system. The
activity of the kidneys, described above, removes any excess of water.
With this state of affairs in tresh-water animals we may contrast that
in the lower (invertebrate) classes of marine animals. These, though
their body walls are more permeable than those ot fresh-water animals,

1 Often called the ureter, although it does not correspond to the
ureter of man.

2 Swelling because the surface of the body of such animals is,
broadly speaking, semipermeable — that is, lets water but does not
let salts pass through it. It it were permeable, as is that of many
marine animals, salts would pass out and the internal fluids would
reach the same concentration as the surrounding water without
swelling. But then the body fluids would be diluted more rapidly.

8o

MANUAL OF ELEMENTARY ZOOLOGY

neither gain nor lose water to any dangerous extent, because the
concentration of salts in the water is approximately that of their
body fluids. About the way in which water is regulated in the bodies
of fishes we shall have something to say later on (p. 448).

Besides urea, the urine of the frog contains smaller quantities of
other nitrogenous excreta. Excretion of most of the nitrogen as
urea is by no means universal among animals. Nitrogen leaves the
protein molecule as ammonia, and in aquatic animals, from which
compounds of ammonium are readily washed out or lost by diffusion.,
such substances form the bulk of the nitrogenous excreta. But these
compounds are highly poisonous, and animals which have not
facilities for immediately getting rid of them must convert the bulk
of their nitrogenous waste into less noxious substances. The frog,
turtles, mammals (including man), and some other animals change
it into urea, which in small quantities is not harmful. Other land
animals (birds, insects, many reptiles) excrete nitrogen in uric acid,
which is both harmless and easily precipitated, so that it can
be voided as a pasty mass and the loss of water thereby much
reduced.

The organs in which the ova and spermatozoa of
animals are formed are known as go?iads .
Reproduction Those in which spermatozoa arise are testes ;

those in which ova arise are ovaries. The male
organs of reproduction of the frog are the testes and their
ducts. The testes are a pair of ovoid bodies slung from the
surface of the kidneys by a fold of the peritoneum known
as the mesorchium. Each consists of a mass of seminiferous
tubules , in which the spermatozoa are formed (Fig. 551).
They communicate by about a dozen small ducts, the 1 rasa
efferentia , in the mesorchium, with the collecting tubules
of the kidney, along which, and through the Wolffian ducts,
the sperm passes to the cloaca, for the male frog has not
separate ducts for sperm (vasa defer entia) and tor urine, but
passes these fluids to the exterior through the same passage.
In the male each Wolffian duct has attached to it a sac,
the vesicula seminalis , in which the sperm is stored until
it is used for fertilising the eggs of the female. In the
female, the ovaries correspond in position to the testes,
the membrane by which each of them is slung being known
as the mesovarium . They are pleated folds of peritoneum
containing ova in various stages of ripeness, each ovum
enclosed in a follicle of smaller cells (p. 114), and all held
together by connective tissue. In the breeding season
they enlarge and shed the ripe ova into the body cavity.

THE FROG : VISCERA AND VASCULAR SYSTEM 81

i.o.d.

r.v.

e.s

r.p.v.

a.ab.v.

odi

ov.v.',

sr.v.

sc.v.

fro.

Fm. 45. — The urinary and genital organs of a female frog.

a.ab.v., Anterior abdominal vein, cut short and turned back ; bl., urinary
bladder ; cl., cloaca ; e.s., egg sac ; f.b., fat body ; f.v., femoral vein ;
2. o.a., internal opening of oviduct ; i.v.c., inferior vena cava ; k., kidney ;
k. d., Wolffian or kidney duct; od.t oviduct; ov., left ovary; ov.v.,
ovarian vein ; pl.v., pelvic vein ; r.l., right lung ; r.v., renal veins ;
r.p.v., renal portal vein ; sc.v., sciatic vein ; sr.b., adrenal body. The
ovary and fat body of the right side have been removed. A diagram of
these organs will be found on p. 82.

6

82

MANUAL OF ELEMENTARY ZOOLOGY

where, by ciliary action, the ova are carried to and into the
internal openings of the oviducts. These are long twisted
tubes, one on each side of the body, opening in tront
into the body cavity by a small aperture at the base of the
lung, and behind into the cloaca just before the opening of
the Wolffian ducts. The greater part of each tube, is
narrow and glandular and secretes a slimy substance, which

Fig. 46.— Diagrams of the urinary and generative organs

of the frog.

A , Organs of the male ; B, those of the female ; bl., bladder ; cl., cloaca ; e.s., egg
sac ; f.b., fat body ; i.o.d., internal opening of oviduct ; k., kidney ; k.d., kid-
ney duct (Wolffian duct) ; od., oviduct ; ov'., ovary ; sr.b., adrenal body ;

testis : v.eff., vasa efferentia ; ves.sem., vesicula seminalis.

sets into a jelly on coming into contact with water, but at
its hinder end the duct enlarges into a sac, which at the
breeding season becomes distended with eggs and occupies
a great part of the body cavity. At this season, which is
in March, the male mounts upon the back ot the female,
clasping her behind the arms with his fore-limbs, which are
provided for the purpose with the pads we have already
mentioned. In this position the animals remain for days
until the eggs are laid. As the spawn passes out, the male

THE FROG : VISCERA AND VASCULAR SYSTEM 83

pours his sperm over it, the eggs are fertilised (p. 13), and
the slimy coating that each of them has acquired in the
oviduct swells up and sets in the water so as to form a
protective layer of jelly. With their subsequent history
we shall deal later (pp. 137, 616).

CHAPTER IV

THE FROG : NERVOUS SYSTEM AND
SENSE ORGANS

Nervous

System

In the nervous system of the frog there may be recog
nised two main parts — the cerebrospinal
Lay-out of the system , connected with the organs ol sense
and the voluntary muscles, and the sympathetic
system , connected principally with the viscera
and blood vessels. The cerebro-spinal system comprises
the central nervous system or cerebro-spinal axis, composed
of the brain and the spinal cord, and the peripheral nervous
system , containing the cerebro-spinal nerves and certain
knots of nerve cells upon them, known as their ganglia.
The cerebro-spinal nerves are ten pairs of cranial nerves
arising from the brain, and the same number ol spinal
nerves. The sympathetic system also consists ol nerves
and ganglia.1

The spinal cord is an elongated, subcylindrical structure,.

Spinal Cord.

lying in the vertebral canal. It is somewhat
flattened from above downward, tapers to a fine
thread, the filum terminale , in the urostyle, and swells
somewhat in the regions of the limbs. A transverse section
(Fig. 49) shows that it is composed of nervous tissue of
two kinds, a grey matter inside and a white matter out-
side (p. 1 16), enclosed in a connective tissue sheath, the-
pia mater , which along the dorsal and ventral middle lines
passes in to some depth as the dorsal and ventral fissures.
In the section the grey matter extends as dorsal and
ventral horns on each side (Fig. 73). In the grey matter
is a longitudinal central canal , which ends blindly behind,
but in front is continuous with cavities in the brain.

The ten pairs of spinal nerves pass out between the
vertebra? to be distributed over the body. Each nerve

1 The sympathetic system, together with some branches of certain
cranial nerves (p. 90), constitutes the visceral system (see p. 93).

84

Fig. 47. — The central nervous system and principal nerves ot a frog,
seen from below. — -Partly alter Ecker.

I.-X., Cranial nerves; 1-10, spinal nerves (see footnote to p. 79) ; V.md., V.mx.,
V.op., mandibular, maxillary, and ophthalmic branches of fifth cranial nerve ;
V.x., a small twig from the undivided main branch of the same nerve ; VI', sixth
cranial nerve after leaving the prootic ganglion ; Vll.h., and VII. md., hvoidean
and mandibular branches of hyomandibular nerve; Vll.hm., VII. pal., hyo-
mandibular and palatine branches of seventh cranial nerve ; IX' , branch from
ninth cranial nerve to seventh ; IX"., main branch of ninth cranial nerve ;
X.v., tenth cranial nerve passing to viscera ; X.x., a branch from the vagus to
certain muscles; an.V., annulus of Vieussens, through which the subclavian
artery passes ; br., brachial nerve ; f.t., filum terminale ; G.g., Gasserian-
geniculate or prootic ganglion ; hy.n., hypoglossal (first spinal) nerve ; inf.,
infundibulum ; pit., pituitary body ; r.c., ramus communicans ; sci.n., sciatic
nerve ; sy.c., longitudinal commissure of sympathetic chain ; sy.g., sympathetic
franelion • i 1 p. vaenis eranerlion.

86

MANUAL OF ELEMENTARY ZOOLOGY

is surrounded as it issues by a soft, white calcareous concre-
tion. Every nerve arises by two roots , a dorsal
spinal Nerves. an(} a ventral, and the dorsal root bears a

small swelling, the dorsal root ganglion. Just outside the
backbone the two roots join, and the nerve thus formed
proceeds at once to divide, giving rise to (a) a short dorsal
branch to the muscles and skin of the back, (b) a long
and conspicuous ventral branch to the muscles and skin of
the sides and ventral surface of the trunk, and in some
cases to the limbs, and (c) a small ramus communicans to
the sympathetic system. The dorsal root is also called
afferent or sensory because along it impulses pass inwards
to the spinal cord and produce, among other effects, sensa-
tion, and the ventral is called similarly efferent or motor
because along it impulses pass outward and produce,
among other effects, contraction of muscles and thus
movements. These functions are proved by the fact that
cutting the dorsal root deprives of sensation the parts
supplied by its nerve, while cutting the ventral root
paralyses the same parts. Each of the branches contains
elements derived from both dorsal and ventral roots. The
course of the dorsal branches and rami communicantes is
much the same in all cases, but that of the ventral branches
differs greatly in different nerves and must now be followed.

The first spinal nerve 1 is known as the hypoglossal.
It leaves the neural canal between the first and second
vertebrae, curves round the throat, turns forward below the
mouth (Fig. 537), and proceeds to the tongue. The second
spinal nerve is a large strand running straight outwards. It
receives branches from the first and third, forming thus the
brachial plexus , and proceeds as the brachial nerve to the
arm. The third spinal nerve is small, and beyond the
brachial plexus resembles the fourth, fifth, and sixth spinal
nerves. All these are small and run backwards to supply
the muscles and skin of the belly The seventh , eighth ,
ninth . and tenth spinal 7ierves join to form a sciatic plexus,
from which arise several nerves to join the hind limb, the

1 The nerve which is counted as the first spinal nerve in the frog is
in reality the second. The true first spinal nerve, which should issue
between the skull and the first vertebra, appears in the embryo, but is
lost later on.

FROG : NERJ'OUS SYSTEM AND SENSE ORGANS 87

principal being the very large sciatic nerve. The tenth
nerve leaves the vertebral canal by a foramen in the side of
the urostyle. The roots of the last four pairs of nerves do
not issue from the spinal canal at once, but run backwards
for some distance from their origin to reach their point of
exit. Thus they form inside the vertebral canal a bundle
known as the cauda equina.

The brain may be divided into three regions, known
respectively as the hind , mid , and fore brains.
The hind-brain consists of the medulla ob-
longata and the cerebellum. The medulla oblongata is the
hindermost part of the brain. It is continuous behind with
the spinal cord, which, as it is traced into the brain, widens,
the central canal enlarging into a cavity in the medulla
known as the fourth ventricle of the brain, the ventral side
thickening, and the dorsal thinning out into a slight mem-
brane over the fourth ventricle (Fig. 30). The pia mater
above this membrane is very vascular and thrown into folds
which project into the ventricle, forming thus a structure
known as th e posterior choroid plexus. The cerebellum is a
narrow band across the roof of the front part of the fourth
ventricle. In many other animals it is relatively much
larger. The mid-brain is the region in front of the medulla.
It has a thick floor formed by two longitudinal columns
known as the crura or pedunculi cerebri, a roof consisting of a
pair of rounded swellings known as optic lobes , and inter-
nally a narrow passage, the aquceductus cerebri, continuous
behind with the fourth ventricle and above with cavities in
the optic lobes. The fore-brain consists of the thalamen-
cepha'lon and the cerebral hemispheres. The thalamen-
cephalon lies immediately in front of the mid-brain. Its
sides are thick and are known as the thalami ; its roof
and floor are thin. The floor is prolonged into a hollow
structure known as the infundibulum , which, with a
glandular, non-nervous mass called the hypophysis ,
makes up the pituitary body. The roof is prolonged into a
short hollow stalk, which in the tadpole is connected with
a structure known as the pineal body. In the adult this
has become separated and lies outside the skull. In
certain other animals the pineal body is much more highly
developed and still connected with its stalk, and its

88

MANUAL OF ELEMENTARY ZOOLOGY

structure shows that it is the remnant of a middle eye,
though it is no longer functional. In front of the pineal
stalk lies an anterior choroid plexus. The cavity of the
thalamencephalon is deep but narrow, and is known as the

Fig. 48. — A diagram of the origin of the spinal nerves of the irog.

cm., Centrum ; d.br., dorsal branch of the nerve ; d.r., dorsal root ; d.r.g., dorsal root
ganglion ; n.a., neural arch ; r.c., ramus communicans ; v.br., ventral branch ;
v.c., vertebral canal ; v.r., ventral root.

dj.

Fig. 49. — A transverse section of the spinal cord of a frog.

c.c. , Central canal ; d.f., dorsal fissure ; d.h., dorsal horn ; g.m., grey matter ;
n.c., large nerve cell ; p.m., pia mater ; v., vein ; v.f., ventral fissure ;

v.h., ventral horn; w.m., white matter. The dorsal and ventral horns are
better seen in the cord of man (Fig. 73).

FROG: NERVOUS SYSTEM AND SENSE ORGANS 89

third ventricle. It is bounded m front by a, wall known as
the lamina termmalis . Behind this on each side an opening
known as XSxcporamen of AI onro or foramen interventriculare
leads into the cavity or lateral ventricle of one of the cere-
bral hemispheres. These are oblong-oval bodies narrowing
forwards to join a mass which is indistinctly separated into
two olfactory lobes. The median walls of the cerebral
hemispheres touch in front and behind, but for a consider-

Fig. 50.— The brain of a frog. — After Wiedersheim.

I. Dorsal Aspect, o.l., Olfactory lobes; c.h., cerebral hemispheres,

P pineal stalk, rising from region of optic thalami ; op.l., optic
lobes ; cb., rudimentary cerebellum ; M.O., medulla oblongata.

II. Ventral Aspect. — The numbers indicate the origins of the
nerves, ch., Optic chiasma ; T.c., tuber cinereum (part of brain floor) •

H., pituitary body.

III. Horizontal Section.— l.v. 1 and 2, lateral ventricles of cerebrum ;
F.m., foramen of Monro ; V .3 and 4, third and fourth ventricles ;

Aq., cavities of optic lobes and aqueduct from third to fourth
ventricle.

able distance they are quite separate. Two regions may
be distinguished in the wall of each cerebral hemisphere—
the ventrolateral region, which is thickened and is known
as the corpus striatum , and the rest of the wall, which is
the pallium. The brain, like the spinal cord, contains
both white and grey matter. Most of the grey matter
adjoins the ventricles as that of the spinal cord adjoins the
central canal, but the grey layer or cortex which in higher
animals overlies the white matter of the pallium (p. 560)
is represented by a rudiment.

90

MANUAL OF ELEMENTARY ZOOLOGY

The first or olfactory cranial nerve of each side arises
from the olfactory lobe and runs forward to
Nerves*1 the °^actory organ in th e nostril. The second
or optic nerve starts from the side of the mid-
brain, curves round underneath the brain, running forwards
and inwards, and crosses its fellow below the thalamen-
cephalon on its way to the eyeball of the opposite side.
Where the nerves cross they are fused, and the X-shaped
structure thus formed is called the optic chiasma .2 The
third or oculomotor nerve supplies the eye muscles, with the
exception of the superior oblique and external rectus. The
small fourth , pathetic , or trochlear nerve arises between the
optic lobe and cerebellum and supplies the superior oblique
muscle. It is the only nerve which starts from the dorsal
surface of the brain. The large fifth or trigeminal nerve
arises from the side of the anterior part of the medulla. Just
before it passes through its foramen it bears a large swell-
ing, the Gasserian-geniculate or prootic ganglion. It then
divides at once into an ophthalmic branch , which runs forwards
along the inner wall of the orbit and supplies the skin of
the forepart of the head, and a main branch , which runs
outwards across the hinder part of the orbit and divides
into a maxillary branch to the upper jaw and a mandibular
branch to the lower jaw. The sixth or abducent nerve is
very small. It arises from the ventral side of the medulla
about the middle of the length of the latter, and supplies
the external rectus muscle, after passing through the
prootic ganglion. The seventh or facial nerve arises from
the side of the medulla behind the fifth. It joins the
prootic ganglion.3 On leaving this it at once divides into a
palatine branch , which runs forwards on the floor of the
orbit to supply the roof of the mouth, and a hyomandibular
branch , which runs outwards and forks into a hyoidean

1 For the foramina by which the cranial nerves leave the skull,
see p. 41. These nerves can more easily be dissected in the dogfish,
where their course is substantially the same (see pp. 452-454).

2 The crossing is not complete, part of each nerve proceeding in that
limb of the X which passes to the eye of the same side.

3 This ganglion is formed by the fusion of two ganglia which are
distinct in the tadpole. One is the Gasserian ganglion and belongs to
the fifth nerve ; the other is the geniculate ganglion and belongs to the
seventh.

FROG : NERVOUS SYSTEM AND SENSE ORGANS 91

branch , to the muscles of the hyoid, and a mandibular or
chorda tympani to the lower jaw. The eighth , auditory,
or acoustic nerve arises from the side of the medulla with
the seventh, enters the auditory capsule, and ends in the
membranous labyrinth of the ear. The ninth or glosso-
pharyngeal nerve arises from the side of the medulla behind
the eighth, immediately joins the tenth nerve, and passes
through the ganglion of the latter. It then bears a small

r 1 * l.CLU \

tha ec. svc

Fig. 51. — A diagram of a dissection, from the left side, of the forepart
of the body of a frog. Compare Fig. 538.

V.-X., Cranial nerves ; V.md., V.mx., V .op., mandibular, maxillary, and
ophthalmic branches of fifth cranial nerve ; Vll.hd., VI I. pal., hyoman-
dibular and palatine branches of seventh cranial nerve ; IX., branch
joining ninth cranial nerve to seventh ; IX"., main branch of ninth cranial
nerve ; X.c., X.g., X.lar., X.p., cardiac, gastric, laryngeal, and pulmonary
branches of tenth cranial nerve ; 1, 2, spinal nerves ; ec., epicoracoid ; Eu.,
Eustachian tube ; hy., hypoglossal ; i.v.c., inferior vena cava ; l.au., left
auricle ; s.v., sinus venosus ; s.v.c., left superior vena cava ; sy., sympathetic
chain ; tr., transverse process of second vertebra ; tr.a., truncus arteriosus;
v., ventricle ; x., xiphisternum.

petrosal ganglion of its own, gives a branch to the hyo-
mandibular nerve, and proceeds round the throat to turn
forward and run along the floor of the mouth, supplying
various structures there. The tenth or vagus nerve is large
and very important. It arises by several roots adjoining
the ninth, with which it is fused as far as the jugular or
vagus ganglion. It then turns backward and downward
round the throat and gives branches to the larynx,
heart, lung, and stomach (Fig. 538). Through the branch

92

MANUAL OF ELEMENTARY ZOOLOGY

which runs to the heart that organ receives from the central
nervous system stimuli which raise or lower the strength
and frequency of its automatic beat. The impulses which
lower the beating arrive through the roots of the vagus from
the brain : those which raise it come through a branch of
the sympathetic system which joins the vagus (see below).

The cranial nerves do not, like the spinal nerves, arise
each by a sensory and a motor root, but it is
Functions of possible to distinguish among them a purely
Nerve^nia' sensory series, a purely motor series, and a series
of mixed function. The tenth, ninth, seventh,
and fifth nerves are ?nixed. They correspond to the dorsal
roots of spinal nerves with that part of the ventral root
(the efferent visceral or autonojnic part) which passes by
the rami communicantes to the sympathetic system. This
part in the cranial nerves passes direct to the viscera and
vascular system. Each of the mixed nerves retains its
dorsal root ganglion as a member of the series formed by the
vagus, glosso-pharyngeal, auditory, geniculate, and Gasserian
ganglia. The sixth, fourth, and third nerves are purely motor
and correspond to the ventral roots of spinal nerves, the
sixth being the ventral root of the seventh, the fourth that
of the fifth, and the third that of a nerve whose dorsal root
is contained in the ophthalmic branch of the fifth. The
eighth is purely sensory and represents part of a dorsal
root. Its ganglion is embedded in the labyrinth of the
ear. The second and first nerves are also purely sensory,
but are not comparable to the dorsal roots of spinal nerves.

The sympathetic system possesses a long nerve-cord on
each side of the body below the backbone and
fystemhetiC alongside the aorta and systemic arch. It is
yS em’ connected by a ramus communicans with each

spinal nerve. At the junction ol each ramus communicans
the sympathetic cord swells into a ganglion. Between the
first two ganglia it is double, becoming thus a loop, the
annulus of Vieussens or ansa subclavia , through which passes
the subclavian artery. In front the longitudinal cord enters
the skull with the "ninth and tenth nerves,, is connected
with the tenth, and ends in the Gasserian ganglion.
From the sympathetic ganglia small nerves are given off
to those of the opposite cord and to the blood vessels and

FROG: NERVOUS SYSTEM AND SENSE ORGANS 93

viscera. These nerves break up and rejoin to form net-
works or plexuses.

It any nerve be traced outward from the central nervous-
system, it is found, after dividing into finer
NervesTn °f and finer branches, to end by entering some
General. organ. Here the fine fibres of which every
nerve is composed (see p. 115) end by coming
into connection with cells in various tissues. Afferent fibres-
(i.e. those traced from the dorsal root or one of the sensorv
cranial nerves) are found to be in relation with cells, known
as receptor cells, of various kinds, which are especially
irritable by some kind of stimulus (as those of the lining of
the eye by light), and their function is to conduct to the
central nervous system impulses set up by these stimuli.
Efferent fibres (i.e. those from the ventral root) join effector
cells — muscular tissue, which the impulses they conduct
will cause to contract, or glandular tissue, which their
impulses will cause to secrete. Thus we may sum up the
arrangement of the nervous system by saying that it
consists of a central mass and a series of afferent and
efferent paths along which impulses pass to and from it.

The nerves ot the cerebrospinal and sympathetic systems are formed
Nerve comPonen*:s which fall into tour primary categories-

Components ' — the somatic afferent , which brings impulses from the

organs of external sense and voluntary muscles ; the
somatic efferent . which carries impulses to the same muscles ; the
visceral afferent, which brings impulses from the viscera and other-
internal organs : and the visceral efferent, which carries impulses to-
the blood vessels, the muscles and glands of the internal organs, and
certain of those ot the skin. As we have seen, each spinal nerve con-
tains components from each of these categories and each cranial nerve
from some ot them only. The sympathetic system has visceral com-
ponents only.

The components of the complete Visceral Nervous System of
vertebrate animals may be tabulated as follows :

(

Visceral System -

Efferent

(autonomic)

Afferent

f Parasympathetic
-< (cranial and sacral)

[_ Sympathetic \ Sympathetic

j Intrasympathetic _f System
\ Extrasympathetic

A nervous impulse is accompanied, and can be traced,
along the nerve by a wave of electrical negativity. It
appears to be an electrical event for which the energy is

94

MANUAL OF ELEMENTARY ZOOLOGY

provided by a chemical process. Both of these are be-
lieved to take place on the surface of the axis
Impulses. i cylinders (p. 1 1 5) of the nerve fibres. In the frog

the average rate at which nervous impulses
travel is 28 metres per second, in man and other warm-
blooded animals it may rise to 120 metres. An impulse is
normally originated at one end of a nerve fibre (as where an
afferent fibre begins in a sense organ) and travels to the
other end, but it can be caused to start at any other point,
and then passes both ways. In either event it travels
without hindrance as far as the points where the nerve fibre
communicates with others (for instance where an afferent
communicates with an efferent fibre) or, if it be an efferent
fibre, with effector cells. The communication between
fibres is made, not by continuity of the fibres, but by an
interlocking of short branches which is called a synapse
(Fig. 68). Through this impulses are able, if they can
overcome a certain resistance which the synapse offers, to
pass in one direction (axon to dendrites, see p. 115 ; for
instance from afferent to efferent fibre) but cannot pass in
the opposite direction. Thus an impulse passes along a
track of fibres in one direction only. Even in that direction,
however, the impulse will not always succeed in passing,
since its ability to proceed depends upon its power of
overcoming the resistance at synapses. This will bar the
excitation generated by a weak stimulus when that of a
stronger stimulus will get through. Actually, a single
nervous impulse, like the contraction of a muscle fibre, is an
“ all-or-none ” phenomenon— that is, if it be produced at
all it has all the strength that it can have, whatever be the
strength of the stimulus that started it. A stimulus, how-
ever, nearly always excites, not a single impulse, but a
series of them, which are more frequent the stronger the
stimulus is. Now as (within certain limits of frequency)
impulses have a cumulative effect which is greater the more
frequent they are, the series of impulses excited by a strong
stimulus is more effective than that of a weak one. The
overcoming of the resistance offered by a synapse is due
to the secretion, by the end of the fibre along which

1 The student is recommended before reading this paragraph to
read pages 115-116.

FROG: NERVOUS SYSTEM AND SENSE ORGANS

95

impulses arrive, of a chemical whose function probably is,
by stimulation, to restart impulses in the fibre on the other
side of the synapse. The action of an impulse at a synapse
is sometimes not to lessen but to increase the resistance.
Thus impulses can inhibit one another. Lastly, it should
be noted that, since nerve fibres branch, an impulse often
has before it more tracks than one, and so, subject to the
factors of resistance and inhibition, an afferent impulse
may through several efferent fibres affect various organs.

It will be clear, from the arrangement of the nervous
system which has been described above, that

fhe'centrai01 ^ *s a complicated apparatus for conveying im-
Nervous System, pulses between the different parts of the body
through the intervention of a central exchange.
In it conductivity is highly developed, as irritability is in.
the sense organs ; its arrangement, however, is such that
impulses are carried, not directly from organ to organ, but
from each organ to the central nervous system, whence,
if action is to take place, fresh impulses are directed to
other organs. It is owing to connections and inhibitions
in the central nervous system that there take place in an
orderly manner the complex responses which the simplest
stimuli evoke in the body of one of the higher animals.
Even such slight and passing actions, for instance, as a
leap from danger or the snapping up of an insect involve
the harmonious action of numerous muscles in a manner
which would be impossible without some mechanism
which will co-ordinate their activity and inhibit that of
muscles which might oppose them.

The actions which are excited through the nervous system
are of two kinds, reflex and voluntary. A
Reflex and reflex action is one in which stimulation of an
Action. afferent nerve brings about through an efferent

nerve the production of activity in some tissue
in an involuntary manner. Thus touching the eye brings
about contraction of the muscle of the eyelid so that
blinking takes place, but this happens without any effort
of the will of the animal, which exercises no choice as to
whether it shall take place or not. In a reflex action the
same stimulus is always followed by the same response.
In blinking the action is conscious, but many reflex actions

96

MANUAL OF ELEMENTARY ZOOLOGY

are purely unconscious, as when the passage of food over
the opening of the bile-duct causes through the central
nervous system a discharge of bile from the gall-bladder
without either the will or the knowledge of the animal.
For a reflex action three things are necessary : (i) an

afferent nerve, (2) a portion of the central nervous system,,
known as the centre of the reflex, (3) an efferent nerve.
This apparatus is known as the reflex arc. For some reflex
actions the centre is in the brain, but for many it is only
necessary that a part of the spinal cord should be intact.
Thus a frog trom which the brain has been removed will,.

af.n Afferent nerve ; c.n.s., central nervous system ; ef.n., efferent nerve ;
mus., muscle ; sen., sensory surface.

if its spinal cord be uninjured, lift its leg to wipe off an
irritant, such as a drop of acid, from its flank. A voluntary-
action is one in which the will intervenes, and a choice is-
made, as when the animal decides between two directions-
in which it can escape an enemy, or wanders to seek food
when it is hungry. Voluntary actions may or may not
follow immediately upon an external stimulus, but when
they do so the same stimulus is not always followed by
the same response. For nearly all, if not for all, voluntary
actions it is necessary that some part of the cerebral
hemispheres should be uninjured.

In the last paragraph we have had to mention conscious-
ness as accompanying certain events in the nervous system.

FROG: NERVOUS SYSTEM AND SENSE ORGANS 97

A conscious being is one that is aware of events. The

Consciousness events which its consciousness immediately
accompanies take place in its own nervous,
tissue. Awareness of other events, within or without the
body, is of course due to nervous events which they cause.1
A vast number of events in the human body are signalled
in consciousness, always, of course, through the agency of
the nervous system. On the other hand, innumerable
events such as the opening of the bile-duct, which we
mentioned above, take place in the viscera and else-
where quite unconsciously, although they affect the nervous,
system. It is, of course,

a

.ch. UA.

A

impossible for us to be cor.

certain that a frog pos-
sesses consciousness, but
just as each of us infers
from the behaviour of other
men that they have a con-
sciousness like his own,
so it may be inferred from
the behaviour of the frog
that it possesses some dim
counterpart of the con-
sciousness of mankind. An
unsolved and perhaps in-
soluble problem is pre-
sented by the relation of
consciousness to the
events which it accom-
panies. Is it caused by

them ? Does it affect their course ? These
course, questions of the highest importance to phil-
osophy. All that need be said here is that, though

FlG. S3 —A diagram of a section
through the eye of a frog.

a.ch ., Anterior chamber; ch ., choroid; cj.p
conjunctiva ; cor., cornea ; ir., iris ;
Is., lens ; lower lid ; o.n ., optic

nerve ; p.ch., posterior chamber ; r.,
retina ; sc/., sclerotic ; u.L, upper lid.

are, of

1 This is not to say that the conscious being knows that the events
of which it is aware are in the first place only events within itself.
Ordinarily, the processes in sense organs and nervous system are not
regarded, and consciousness is accepted as first-hand evidence ot
external things. Still less is it realised that consciousness does not
necessarily present the likeness of things outside it, not even of the
bodies with whose working it is linked, but rather signs which stand for
such things, and that after this fashion alone does it know material
things.

7

•98

MANUAL OF ELEMENTARY ZOOLOGY

the consciousness of the living organism is probably
always accompanied with events in the nervous system,
the legitimate inference from this is, not that either
of them is the cause of the other, but rather that
between them there is some relation whose nature is
unknown to us. We are here dealing with two things of
wholly different kinds, the events which happen in the
nervous system being physical 1 processes and conscious-
ness a psychical process. The difficulty in imagining any
interaction between them lies in the fact that such action
would apparently be contrary to the principle of the
conservation of energy, though this has been denied.
On the other hand, it seems equally clear that conscious-
ness could not be talked or written about, or, if all
thinking be dependent upon processes in the brain, even
thought about, unless it affected the nervous system.

The senses of a backboned animal, such as a frog, are
more numerous than is generally realised,
sense organs : Besides the “five senses” of sight, hearing,
ments. " smell, taste, and touch, there are distinct kinds
of sensibility to heat, cold, and the movements
of the body, and an indefinite “ general sensibility ” which
when it is slight escapes attention, but when it becomes
excessive rises into pain. Each of these senses has origin
in impulses derived from a special kind of nerve ending,
but only in the case of sight, hearing, and smell are these
endings situated in a highly specialised organ. We shall
here consider only these organs.

The eyeball of the frog is roughly spherical, but flattened
Eyes on the front side. It consists of the following

parts : (i) The outer coat or sense capsule corre-
sponds to the auditory and nasal capsules, but fits closely to
the eye instead of forming a hollow capsule fused to the
skull. Over the greater part of the eye it consists of dense
connective tissue with some cartilage and is known as the
sclerotic , but on the front side it is transparent and known
as the cornea. (2) The skin over the cornea adheres to it
as a delicate, transparent covering, the conjunctiva , which
is kept moist by the secretion of Harderian glands below

1 That is, processes which go on in material bodies ; not necessarily
physical ” in the sense in which that term is opposed to “ chemical.”

FROG: NERVOUS SYSTEM AND SENSE ORGANS

99

the eye. (3) Inside the sense capsule is the choroid coat ,
consisting ot looser and highly vascular connective tissue
containing numerous dark pigment cells. In front the
choroid separates trom the sclerotic and passes inwards,
as a partition called the iris , across the hollow of the
eyeball, which it thus divides into anterior and posterior
chambers. The former is smaller and filled with a watery
aqueous humour , the latter larger and filled with a
gelatinous vitreous humour. In the middle of the iris is
an opening, the pupil , and the iris contains muscular tissue
by which the size of the pupil can be altered. (4) Im-
mediately behind the iris lies a firm, transparent, sub-
spherical body, the lens , which serves to focus upon the
sensitive surface at the back of the eye the light which
enters through the pupil. (5) The sensitive surface is
provided by the retina , a delicate membrane containing
two primary layers, an outer pigment layer of pigmented
cells lining the choroid, and an inner retina proper which
has at the back of the eye a very complicated structure
(Fig. 61, B) and is connected with the optic nerve, by
which the impulses which give rise to sight are conveyed
to the brain. The fibres of the optic nerve pass right
through the retina and spread out over its inner surface
(that which is turned towards the hollow of the eyeball).
The percipient cells are on the outer surface, against the
pigment layer, so that light must pass through the layer
of nerve fibres to reach them. In the front half of the eye
the retina loses its complicated structure and becomes very
thin, but it continues to line the posterior chamber up to
the edge of the pupil.

For an object to be seen it is necessary that an image of
it should be formed on the retina. The formation of such
an image is due mainly to refraction of light by the lens,
but refraction also takes place at the surfaces of all the
other media (cornea, aqueous humour, vitreous humour)
through which the light passes in the eye. In the diagram
below (Fig. 54) these effects are combined and the refrac-
tion shown as taking place at a single surface (p) situated
in the aqueous humour. Each point of the object may be
considered as sending out a pencil of divergent rays which
by refraction are made to converge again into a point in

IOO

MANUAL OF ELEMENTARY ZOOLOGY

an image which is constituted by such points corresponding
to those of the object. The diagram shows that the image
is inverted — what is the upper part of the object is repre-
sented in the lower part of the image and what is on the
right-hand side of the object by the left-hand side of the
image. In the judgment which is based on the visual
sensation, however, allowance is made for this — the left-
hand side or the bottom of the image is taken as a token
of the right-hand or top of the object.

Fig. 54. — A diagram to show the formation of a retinal image.

a, b, c, Rays proceeding from the point X ; a', b', c' , rays proceeding from the-
point Y ; p, theoretical “ principal surface,” at which the combined
refraction caused by the several surfaces of the eye is supposed to
take effect. The lens is shaped as in man and the rabbit.

The visual impulse originates in the retina, in the “ layer of
rods and cones” (p. ill). The rods, which are stimulated by a
photochemical change in a substance known as visual purple that
they contain, give vision in dimmer light than that which is necessary
to stimulate the cones. The latter, owing to certain features of their
nervous connections, are more acute in perceiving fine detail than
the rods ; they also provide colour vision in those animals which
possess it. The image is focussed in the frog and in fishes by moving-
the lens to and fro, in man and the rabbit by altering the convexity
of the lens.

The essential part of the ear is the membranous labyrinth
Ears which we have already mentioned (pp. 42, 91).

It lies in the cavity of the auditory capsule.
This cavity contains a fluid known as perilymph , and the
membranous labyrinth contains a fluid known as endo-
lymph. The labyrinth consists of the vestibule and the
semicircular canals. The vestibule has an upper, larger
division, the utriculus , and a lower, smaller sacculus .
From the former arise the three semicircular canals, which
are arched tubes opening into the utriculus at both ends.
They are placed in planes at right angles to one another,.

FROG: NERVOUS SYSTEM AND SENSE ORGANS ioi

. v.s .

p.V.S.

•one of them being horizontal, another longitudinal-
vertical (the posterior vertical ), and another transverse
vertical (the anterior vertical). One of the ends of each
of them is enlarged to form a small, rounded ampulla.
From the sacculus arises an offshoot known as the lagena
which has three small dilatations ; this represents the
cochlea of higher animals. On the median side of the
sacculus there starts a tube, the ductus endolymphaticus ,
which enters the cranial cavity and there expands into a
thin-walled saccus. Be- uty.

tween the auditory |

•capsules and the mem-
brane of the ear-drum
or tympanic membrane
on the side of the head
lies the cavity of the
car-drum or tympanic
cavity , which, as we
have seen, is crossed
by the columella from
the fenestra ovalis to
the tympanic mem-

brane and communi-
cates with the pharynx

by the Eustachian

tube. This region is
called the middle ear,
the labyrinth being the
inner ear. There is no
outer ear in the frog.

The semicircular canals are not organs of hearing, but
enable the animal to keep its balance by
Functions of judging the position of its head. Placed as
they are m three planes of space, the ttuicl m
them is set in movement by any change in position, and
the differences in pressure on their walls which are thus
brought about start impulses which the auditory nerve
conveys to the brain. When they are diseased or injured
giddiness is caused. The true organ of hearing is the
sacculus. The vibrations which constitute sound set the
tympanic membrane in motion, and its movements are

mup.

Pig. 55. —The labyrinth of the right ear
of the frog, seen from the outer side.
— Partly after Marshall.

11.71.S., Anterior vertical semicircular canal;
amp ., ampullae ; chi., small dilatations of
the sacculus which represent the cochlea
of higher animals; h.s., horizontal semi-
circular canal ; n., branches of the auditory
nerve to supply the ampullae ; p.v.s., pos-
terior vertical semicircular canal ; sac.,
sacculus; utr., utriculus.

102

MANUAL OF ELEMENTARY ZOOLOGY

Olfactory

Organs.

transferred by the columella to the membrane of the
fenestra ovalis and thence through the perilymph and the
wall of the membranous labyrinth to the endolymph, where

'peril they stimulate the
endings of the audi-
tory nerve in the dilata-
tions of the saccule.
The organs of smell
are a pair
of irregu-
lar cham-
bers, enclosed by the
nasal capsules, separ-
ated by the nasal
septum , and com-
municating with the
exterior by the nostrils
and with the mouth
by the internal nares.
The lining of each is
connected with the
olfactory nerve of its
side. Air is drawn through the chambers in the process
of breathing, and the odorous particles it contains affect
certain cells of the lining which are connected with fibres
of the nerve.

E%i

Fig. 56. — A diagram of the ear of the frog.

col., Columella ; f.o., fenestra ovalis ; Eu., Eusta-
chian tube ; lab., part of the membranous
labyrinth, containing endolymph ; m., mouth ;
md., mandible ; peril., perilymph ; sk., skull ;
tym., tympanic membrane.

CHAPTER V

THE FROG: HISTOLOGY, THE GERM
CELLS, DEATH

The study of tissues is a branch of anatomy known as
Histology. It was shown in the first chapter
Cei|t8.,0gy that tissues of the animal body consist of
protoplasm accompanied in many cases by a
ground-substance which supports it. The differences between
tissues depend upon differences in arrangement and com-
position both of the protoplasm and of the ground-substance.
When protoplasm is stained with certain dyes, a portion of
tt colours more readily and deeply than the rest. This
portion is usually collected into minute masses known as
nuclei. The matter of which the nuclei are composed is
known as nucleoplasm , the rest of the protoplasm as cyto
plasm. In most tissues 1 the protoplasm is arranged in the
little divisions, known as cells, to which wTe have already
alluded, each cell containing a nucleus and being of a size
and shape peculiar to the tissue to which it belongs. The
cells may either lie side by side (Fig. 8, C) or be separated
by ground-substance (Fig. 8, B) or by fluid, as in the
tissue known as blood (Fig. 8i). Every cell arises by
fission from another cell, grows, and behaves to some
extent as an independent individual, but in the majority
of cases it remains in connection with its neighbours by
fine strands of protoplasm. A cell-like unit with more
than one nucleus is known as a ccenocyte , or sometimes as
Protoplasm itself has a fine structure which varies from
D . . tissue to tissue. Under high magnification it

usually appears homogeneous, but sometimes
— when killed, usually — it shows a meshwork composed of

1 In all the tissues of the frog.

103

io4 MANUAL OF ELEMENTARY ZOOLOGY

.a denser substance known as spongioplasm with a more
fluid enchylema or hyaloplasm in its interstices.1 Both
these substances are liquids containing various substances
in solution and others in suspension as granules and drop-
lets. Of the chemical composition of these solutions only
the broad outlines are known, since it is not possible to
analyse protoplasm without killing it and thereby bringing
about in it chemical changes. The solvent is water, and in
dead protoplasm the dissolved substances are found to be
in part inorganic salts, such as the phosphates and chlorides

Fig. 57. — A diagram of a cell, after it has been killed and stained.

chi'., Chromatin granules; gr., granules in cytoplasm; h.,
linin meshwork of nucleus ; nu.m., nuclear membrane.

•of sodium, potassium, and calcium, but principally organic
•compounds, and those mainly colloidal. Some of the
•organic bodies, such as glycogen and organic compounds

1 This appearance has been interpreted in various ways. It is
-probably due, at least in many cases, to the fact that such protoplasm
which exhibits it is a foam or emulsion, the walls of whose bubbles
are formed by the spongioplasm while the enchylema fills them.
These minute bubbles must not be confused with the larger spaces
known as vacuoles. The spongioplasm may contain threads or
.granules of still denser living substance. Its ultimate framework
probably consists of loosely interlacing linear protein molecules or
molecule-chains.

THE FROG : HISTOLOGY, GERM CELLS, DEATH 105

of ammonia, are comparatively simple ; these were pro-
bably in course of assimilation or excretion by the living
substance. The greater part, however, consists of pro-
teins, which are peculiar to protoplasm and never found
except in it or in substances manufactured by it. Meta-
bolism never occurs without proteins, and its peculiar
features certainly depend largely upon the nature of these
substances, but it must not be overlooked that metabolism
is exhibited only when the proteins form part of proto-
plasm, and only during the life of the latter.

Two physical facts are of great importance for an
understanding of the properties of protoplasm. The
first of these is surface action. Owing to the mutual
attraction of its molecules, the surface layer of a body of
liquid behaves like a stretched membrane. In this, for
reasons into which we cannot enter, there occur various
actions, chemical, physical, and mechanical, which take
place less readily or not at all in the interior. One of
the most important of these is adsorption , by which
•certain substances are attracted into the surface film,
there to be retained, to enter into reactions with other
•substances, or to be passed into solution on the other side.
In the living body, surface action takes place both on the
surfaces of cells and on those of structures within the
protoplasm, such as vacuoles ; and it is a factor in a
vast number of processes, notably in the keeping in and
excluding, taking up and casting away of substances by
the protoplasm, in the formation of actual membranes,
.and perhaps in contraction. The second fact is the
existence of certain properties of colloid solutions. Sub-
stances in solution are either “ colloid ”or“ crystalloid 1 ;
and those in the former of these conditions differ from
those in the latter in that they do not readily crystallise,
do not pass through membranes, and diffuse only very
slowly. The size of their particles is responsible lor
these and other features of colloidal solutions. Though
too small to sink, the particles are large enough to exhibit
surface action of their own. Hence (a) in certain circum-
stances they abandon their condition of solution (known
as a “ sol ”) by uniting into a more or less firm jelly or
<l gel,” which may be reversible, as is that ol gelatin by

io6

MANUAL OF ELEMENTARY ZOOLOGY

msnt.

... c. m.

heating or cooling ; (b) they tend, when they are attracted
into the surface film, there at times to unite into a true
membrane such as that which forms upon hot milk
when it cools ; (V) owing to adsorption by their surfaces,
the solution as a whole has a great power of taking up
substances. We have seen that protoplasm contains a
great quantity of colloids. It is owing to these that it

does not even when naked lose
its identity by diffusion into the
surrounding liquid, can form
membranes upon its surfaces,
can build internal structures
by the formation of gels, and
can take up and hold great
quantities of materials from
solutions around it.

The granules and droplets
suspended in protoplasm vary
in composition and function.
Some of them are truly alive.
They are probably of the same
nature as the spongioplasm but .
Fig. 5^. — A diagram of a denser. Others, which may con-
transverse section through sist of proteins, carbohydrates,
the ileum of the frog. or fats? are to be regarded as

Circular muscle layer ; c.t ., material removed, for a time at

wh ' c hC f i' n e s' Th e' g ! u ^ ngi- least, from metabolism. Besides

tudinal muscle layer; msnt., these bodies protoplasm often

rid., longitudinal ridges of ileum, contains relatively large spaces

filled with fluid known as vacuoles.

Protoplasm is extremely sensitive to the action of ex-
ternal agents, from which it is carefully screened
Protoplasm.* in the bodies of most animals. Distilled water,
solutions of salts, acids, alkalis, and narcotics
all have characteristic effects upon it, stimulating, in-
hibiting, or killing it according to their nature and strength.
It is also easily stimulated by electric shocks.

Changes of temperature have marked effects upon it.
Moderate heat acts as a stimulus, causing increased
activity of movements, etc. ; cold has a depressing effect.
Heating above about 450 C. kills the protoplasm by

rid.

THE FROG: HISTOLOGY , GERM CELLS , DEATH 107

causing its proteins to set into a solid mass or “ coagulate.”
Cold below o° C. stops all exhibition of life, but unless it is
extreme does not prevent the reappearance of vital pro-
cesses if the temperature be raised.

?.c. let.

/

Fig. 59. — A portion of the section shown in Fig. 53, more highly

magnified.

b v.. Blood vessel ; c.t. , connective tissue of mucous membrane ; c.ni., circular layei
of muscle fibres ; ep., epithelium ; g.c., goblet cell; Lin., longitudinal layer o
muscle fibres ; let., “ lacteal ” or lymph vessel of the intestine ; leu., leucocyte
of lymph or lymph corpuscle \ p.e., peritoneal epithelium.

Protoplasm is, as we have said, the living part of the
body. It is here that metabolism goes on and all the
characteristic processes of life take place. The reception
of stimuli, conduction, contraction, secretion, reproduction,
and so forth take place in the protoplasm alone. The rest

MANUAL OF ELEMENTARY ZOOLOGY

308

•of the body exists only to support and protect it. Life is

the life of protoplasm.

It is sometimes attempted to recognise in protoplasm a
truly living part which undergoes relatively little change
.and a part which is not truly alive, but undergoes the
■changes which provide the bulk of the life energy, under
the influence of the living part acting as a kind of ferment
<p. 61). No doubt it is the case that there are many
grades of metabolism, but in the broad view it is better
to regard protoplasm as a single complex mixture ot
.-substances in which there go on the chemical changes
■which are the basis of the process we know as life.

The cytoplasm nearly always contains certain other organs as well
as the nucleus. Under the surface film whose functions
Cy topiasm thC we have noted there usually lies a viscid cortical layer
which gives the cell its shape. Internally there are
certain liquid entities, often rod-shaped, known as mitochondria and
some strands or spherules called Golgi bodies both ot which pertorm
some unascertained functions in its chemical activity, and usually
also there is a protoplasmic sphere known as the centrosome which,
;as we shall see, plays an important part in cell division.

When it is “ resting ” (i.e. not dividing) the nucleus is
enclosed by a delicate nuclear membrane and
if it be killed appears to consist of a viscid

imeshwork of linin with a thinner nuclear sap in its
interstices. In the linin there lie granules of the deeply-
staining substance chromatin 1 which at division become
•evident, both in living and in killed nuclei, in the form
•of the bodies known as “ chromosomes ” (p. 129). There
.are also in the resting nucleus bodies known loosely as
nucleoli which may consist of chromatin or not, are
variously stainable, and have various but unascer-
tained functions. The nucleus has been experimentally
proved, by operations under the microscope in which
it was removed from the cytoplasm, to be necessary to
The continuance of life : without it many vital processes,
;such as fission — which is always preceded by division ot
the nucleus — assimilation, and secretion, are impossible,
.and life soon comes to an end. Also, as we shall see, it

1 Chromatin consists of various compounds of the class known as
nucleins. Nucleins contain protein united with another highly complex
•substance known as nucleic acid, very rich in phosphorus. Its
.acidity causes it to unite with basic dyes.

THE FROG : HISTOLOGY, GERM CELLS, DEATH

IOQ>

plays an extremely important part in heredity. From
these and other facts it appears that the nucleus has a
regulative action over the life of the protoplasm. But the
nucleus is no more capable of life apart from the cytoplasm
than the latter can live without nucleoplasm. Thus the-
unit of living matter is a portion of nucleoplasm with its.
accompanying cytoplasm. Such a unit is known as an.
energid. A cell is an energid which is in some way de-
limited from the rest of the energids which with it form the-
body of an organism. A ccenocyte is a group of energids.

Every tissue belongs to one of four classes : it is either
Kinds of epithelial, skeletal, muscular, or nervous. The

Tissues : epithelial tissues are those which cover surfaces.

Tipssuheesia< internal or external. They consist of cells of
simple shape arranged to form a layer, with
little or no ground-substance between
them. \\ hen the cells are one laver
deep the epithelium is said to be
simple ; when there is more than
one layer it is stratfied. Perhaps
the least specialised example of this
class ot issue is the kind known
as columnar epithelium, found, for
instance, lining the intestine of the
frog (Figs. 58, 59). This is a simple
epithelium, consisting of one layer of
tall cells standing side by side like
columns. Between the cells exist exceedingly fine
crevices which communicate below with lymph-spaces,
and across the crevices the protoplasm of the cells is,
continuous as fine threads. A modification of this kind
of epithelium, known as ciliated epithelium, is found
on the root ot the mouth ot the frog. Here the outer
border ot the cell is set with very fine protoplasmic hairs
known as cilia , which are in constant lashing motion
in one direction. As they bend sharply and recover
slowly, the effect of their combined action is to drive the
fluid which covers the epithelium in the direction of their
lashing. (Fig. 95 D). From each cilium a fine thread runs
down into the cytoplasm of the cell. A third modification
of columnar epithelium is the kind known as sensory. In

Fic, 60. • — Isolated cells
of ciliated epithelium,
from the roof of the
mouth of a frog.

Fig. 6i. — Examples of different modes of ending of sensory nerve

fibres of the frog.

A, Cells from the olfactory epithelium. B, cells from the retina, much simplified.
C, cells from one of the patches of sensory epithelium in the labyrinth, with
which the fibres of the auditory nerve are connected. D, a portion of the
epidermis, showing the ending of a nerve fibre.

D is ordinary stratified epithelium. A, B, and C are true sensory epithelia — forms of
columnar epithelium adapted to the purposes of special senses. In these latter
there can be distinguished sense cells and supporting cells. The sense cells bear
pr ocesses of various kinds on the surface of the epithelium, and at their other ends
connb into relation with nerve fibres., In A the sense cell is prolonged into a fibre
which runs in one of the olfactory nerves as a non-medullated nerve fibre (p. 94).
In B also the sense cells are prolonged into fibres though these are connected
with the nerve by the intermediation of other cells with whose processes their fibres
interlock. In C, on the other hand, the sense ceils are not continued into fibres,
but are embraced by branches of nerve fibres belonging to cells in the ganglion of
the auditory nerve. Thus they resemble D, where the nerve fibres have a similar
relation to the cells of the epithelium. In the lower animals, such as the
earthworm, the sensory nerve endings in the skin are usually of the type of A
and B, rather than that of C and D.

cn. C>..ne; n.c., nerve cells; »./., nerve fibres; rd., rod; s.c., sense ceils; tt.c..
supporting cells.

no

THE FROG : HISTOLOGY , GERM CELLS, DEATH hi

$t. 1.

this some or all of the cells bear at the outer end one or
more stiff processes, the size and shape of which vary
greatly in different cases. Each such cell is connected
with a sensory nerve, either by being itself prolonged
internally into a fibre, which runs in the nerve, or by such
a fibre ending against it. Cells of this kind are found,
for instance, in the olfactory epithelium, where each bears
a tuft of stiff bristles, and in the
retina, where each ends in a stout
rod-shaped or conical body. These
compose the layer of “ rods and
cones ’’ which lines the retina.

Glandular epithelium is a kind
of simple epithelium in which
the cells have become specialised
for the secretion of chemical sub-
stances. It may occur as single
cells scattered among those of
ordinary columnar epithelium.

This is seen, for instance, in the
intestine of the frog, where some
of the cells store at their outer
ends granules of a substance
which, when they discharge it,
forms the slimy mucus which
gives the lining of the alimentary
canal and other spaces the name
of mucous membrane. After the
discharge of this substance there
is left a cup-shaped hollow in
the cell, on which account it is
called a goblet cell. The hollow
is presently filled again by the
activity of the protoplasm of the cell. Isolated gland-cells
in an epithelium are sometimes known as unicellular glands.
Collections of gland-cells form multicellular glands. The
simplest kind of these is found in the mucous membrane of
the stomach. The epithelium here dips down into the
underlying connective tissue as hollow tubular processes
like the fingers of a glove. The mouths of these tubes are
lined with ordinary columnar epithelium which deeper in

Fig. 02. — One of the glands
of the frog’s stomach.

7., Duct ; /., the secreting part of
the gland, known as the
fundus or alveolus ; st.L, epi-
thelium lining the stomach.

1 12

MANUAL OF ELEMENTARY ZOOLOGY

the tube is succeeded by somewhat lower cells. This
region is the duct (p. 6) of the gland. At the end of
the tube the cells are large and more nearly cubical and
contain in their protoplasm granules of a substance
which, when it is discharged, forms the enzyme of the

D

Fig

63. — Diagrams of different kinds of
glands. — Partly after Lang.

A , Columnar epithelium containing isolated gland cells
or unicellular glands. B, similar epithelium with
the gland cells collected into a group so as to form
a flat multicellular gland. C, a hollow multi-
cellular gland of the simple kind. The figure
represents a type intermediate between the sac-
cular glands of the frog’s skin (Fig. ,60) and the
tubular glands of the frog’s stomach (Fig. 57).
The latter, however, may be forked, and thus
show a transition to D, the compound or race-
mose glands.

al., Alveoli or acini of the racemose gland; d., ducts;
/., alveolus or fundus of simple gland; g.c., gland
cells.

juice secreted by
the gland.1 The
granules do not
leave a hollow in
the cell when they
are discharged.
Such a gland is
known as a tubular
gland. The pan-
creas is an example
of the more compli-
cated class known
as racemose glands ,
in which the tubes
are branched and
lined with low,
cubical epithelium
up to their ends,
which are dilated
and lined with
glandular epithel-
ium. The dila-
tions are known as
acini and the tubes
leading to them are
ducts. The liver is
more complicated
still, the tubes not

only branching but rejoining to form a meshwork, whose
walls consist of gland cells. Pavement epithelium also
belongs to the simple class, but is very different from any
of those we have seen hitherto. In it the cells are flat,
and so thin that their surface is raised where the nucleus
lies. They are separated by narrow but distinct lines of

1 The granules themselves consist not of the enzyme but of a
precursor called the zymogen.

THE FROG : HISTOLOGY, GERM CELLS, DEATH

n$

intercellular substance which stains strongly with silver-
nitrate, and the surface has then the appearance of being-
composed of flat tiles, like a pavement, from which cir-
cumstance the name of the tissue is derived. The coelom,
blood vessels, and lymphatic vessels are lined with this-
epithelium.1 Stratified epithelium consists of several
layers of cells. It is found in the epidermis or scarf skin
which forms the surface of the frog’s skin. In it the
lowest layer consists of deep cells with unaltered proto-
plasmic bodies, but
successive layers
from within out-
wards become more
and more flattened
and converted into
a horny substance
till the outer layer
consists of flat,
horny scales which
are shed, being re-
newed from within
by the division of
the lower layer.

The inner, softer
strata are known
as the Malpighian
layer. Germinal
epithelium consists
of columnar or
cubical cells with
rounded cells derived from them, some of which give rise to>
ova and spermatozoa. It is found lining the seminiferous
tubules of the testes (Fig. 538) and covering the surface of the
ovaries. Ova and spermatozoa have each a single nucleus.
The spermatozoa are minute structures consisting of an
elongated head , which contains the nucleus in a very thin
investment of cytoplasm, a short neck , which consists of
protoplasm containing a centrosome (see p. 128), and
a tail, which has the form of a flagellum or lash of

1 See Figs. S A (surface view) and 59 (section, showing surface-
raised by nucleus).

8

y.c.

Fig. 64. —A small portion of a section of a
frog’s liver, highly magnified.

b.v., Blood vessel ; l.c., liver cells; lunt. , lumen of
liver tubes ; r.c., red corpuscles.

1 14

MANUAL OF ELEMENTARY ZOOLOGY

protoplasm. The ova are large, rounded cells containing
numerous granules of food matter or yolk and blackened

Fig. 65. — A section of the skin of a frog, taken vertically to the

surface, highly magnified.

b.Vo, Small blood vessels; cap., capillaries; d.l, dense layer of connective tissue,
consisting of fibres which lie parallel to the surface , der ., dermis or corium ; <?/.,
epidermis; gt., gl" ., gt" ., glands of three kinds; gl' . and gi" . secrete a slimy
mucus and pass it to the surface of the skin by ducts which are not shown in the
section ; gl" . secretes a more watery secretion which probably contains a
substance of unpleasant taste ; all three kinds are simple glands of the saccular
type; h.l. , horny layer of the epidermis; M.l. , lowest row of the Malpighian
layer of the epidermis ; pig., pigment cells ; vf, strands of vertical fibres in the
connective tissue.

on one side by the presence of pigment in the surface
layer. On this side lies the nucleus. There is no centro-
some. Each ovum is enclosed in a vitelline membrane ,
and surrounded in the ovary by a case ox follicle of cells,

Nervous

Tissue.

THE FROG : HISTOLOGY , GERM CELLS , DEATH n5

derived from the germinal epithelium, which manufacture
the } oik and pass it to the ovum.

Nervous tissue consists of cells provided with processes
lor the purpose ol conducting impulses. In
each such cell or neurone (Fig. 67) there may
. . be distinguished (1) a cell body. , containing

the nucleus, _ (2) a long process known as the axon
along which impulses are discharged, (3) other processes,’
usually short, numerous, and highly branched, known as
the dendrites , along which
impulses reach the cell.

The axon ends by breaking
up into a tuft of branches,
the terminal arborisation ,
irom which a stimulus is
given either to another
nerve cell (Fig. 68) or to
the elements of some
different tissue, as for in-
stance to one of the fibres
which compose muscle
(Fig. 69 C). The axon is
often of very great length,
and is then known as a
nerve fibre , the term “ nerve 3Cale'

cell ” beinp- onrdiprl tn he- follicle of germinal

:: . , T appnea to Its epithelium cells around ovum; A.,

Cell body alone. Such a head; nu., “germinal vesicle,” or

fibre _ bears at intervals ; p'ri'

nuclei which, however, do

not belong to it, but have a small amount of granular
cytoplasm of their own. They are called nuclei of the
sheath, because they underlie and appear to belong to a
dedicate, membranous primitive sheath or neurilemma
which generally covers the fibre. Nerve fibres are of two
kinds, medullated and non-medullated. In the former a
layer ol fatty substance, known as the medullary sheath ,
lies between the neurilemma and the axon, which is called
the axis cylinder. The medullary sheath is broken at
m ter va s at spots known as nodes of Ranvier , and the
nuclei are arranged one to each internode. In non-
medullated fibres the fatty sheath is wanting. Medullated

l1 ig. 66. —Germs of the frog. A ,
spermatozoon ; B , unripe ovum
in the ovary ; C, ripe ovum.
Magnified, hut not drawn to
scale.

MANUAL OF ELEMENTARY ZOOLOGY

1 16

d.

fibres are found principally in the cerebro-spinal system,,
non-medullated fibres principally in the sympathetic
system. Nerve fibres may branch.

The grey matter of the central nervous system consists-
of cell bodies
embedded in an
exceedingly fine
network composed
of dendrites and
terminal arborisa-
tions, with fibres

passing through it on
way from the cell bodies,
white matter and the

their
The
nerves*

Fig. 67. — Diagram of part of
a neurone highly magnified.

ax ., Axon ; c.i., cell body ; </.,
dendrites ; nu., nucleus.

consists of fibres only. Ganglia
consist of cell bodies inter-
spersed with fibres, some or
all of which make connection-,
with the cells. In the dorsal
root ganglia each cell has only
one process, and this after’
running for a short distance-
divides at right angles, giving
off in one direction an axon
which enters the central
nervous system, and receiving
in the other a process ( dendron )
which has the structure of a
a dendrite, bringing impulses

fibre, but. functions as
inwards.

Muscular tissue consists of elongated elements, either
cells or ccenocytes, the muscle fibres , in which the power

THE FROG: HISTOLOGY, GERM CELLS, DEATH 117

of contraction is highly developed. It is of two kinds,

Muscular striped or striated \ and plain , unstriped or

Tissues. unstriated. Involuntary muscle (see p. 49)

is generally unstriped, voluntary muscle striped.
The fibres of plain muscle are cells of an elongated

Fig. 68. — A diagram of fibres entering and leaving the spinal
cord, showing various tracks along which impulses may be
conducted in the “ exchange ” system which it constitutes
(p. 95). The arrows show the direction of impulses.

-Note that where the terminal branches of the axon of one
neurone meet the dendrites of another the two are not
continuous but interlace, so that the nervous impulse must
pass an interruption in its track. This arrangement is
called a synapse.

1. The simplest track, one afferent and one efferent neurone ; 2. an
intermediate neurone is concerned ; 3. the track crosses the cord, so
that an impulse is discharged along a nerve on the opposite side ; 4. a
neurone receives impulses from two others ; 5, 5', 5". a fibre branches
to affect neurones in different parts of the cord ; d.r.g.c. cells of dorsal
root ganglion.

•spindle shape with a single oval nucleus in the middle.
They show a faint longitudinal striation, have each a
delicate sheath, and when they occur in masses are joined
by a little intercellular cement substance, across which
threads of protoplasm keep them in continuity (Fig. 8 B).
The fibres of striped muscle are larger than those of plain
..muscle, and can be seen with the naked eye if a portion

ii8 MANUAL OF ELEMENTARY ZOOLOGY

ol one of the great voluntary muscles be teased into frag-
ments. Each is a ccenocyte, containing many nuclei. The
nine is cylindrical, tapering at the ends. It is covered by

Fig. 69- — Histology of muscle.

A, Cardiac muscle of the frog ; B, cardiac muscle of man ;

C, part of a skeletal muscle fibre of the frog, showing the
ending of a motor nerve fibre.

ax.c., Axis cylinder of the nerve fibre ; br., branches of the same
after it has lost its medullary sheath : n.br.y nuclei of the
branches , w,f.9 nuclei of the nerve fibre i fi.wi., nuclei of
muscle fibres.

a transparent elastic membrane, the savcolefyivta^ which
adheres at the end to that of an adjacent fibre or to tendon.
The cytoplasm is characterised by alternate light and dark
stripes which lie across the length of the fibre (Figs. 69, 77).

THE FROG : HISTOLOGY , GERM CELLS , DEATH 119

Skeletal

Tissues.

substance

-n.R.

m. s.

f -nl.

a x. c.

lit

_ n.u.

m

The nuclei are scattered throughout the substance of the
fibre, and often each is surrounded by a little granular
undifferentiated protoplasm unlike the modified proto-
plasm which composes most of
the fibre. A longitudinal fibrilla-
tion is present, but its relation
to the striation is a matter of
dispute. The muscular tissue
of the heart is a peculiar kind of
striped muscle known as cardiac
muscle. Its cells, spindle-shaped
in the frog, short, square-ended
cylinders in Man, have each
one nucleus, and often a branch
that abuts upon a similar process
of a neighbouring cell. It is
striped, but less distinctly so
than ordinary muscle. It is in
this tissue that the automatism
of the heart resides.

Skeletal tissues are character-
ised by a great de-
velopment of inter-
cellular or ground
between the cells
whereby it is adapted to sup-
port and bind together other
tissues. In cartilage the ground
substance is a homogeneous
matrix readily stained by silver
nitrate. It is firm and elastic,
and sometimes toughened by
the development in it of fibres,
when it is known as fibrous
cartilage , in contradistinction to
plain or hyaline cartilage. The
cells are simple in shape and
often disposed in groups of
two or four formed by the
division of one original cell. As
these secrete additional ground

Fig. 70. — Medullated nerve
fibres, stained with osmic
acid and highly magnified.

a.x.c., Axis cylinder ; m.s. , med-
ullary sheath ; nl., neuri-
lemma ; n.R., node of

Ranvier ; nu., nucleus.

Note that the axis cylinder con-
tains longitudinal fibrils.

320

MANUAL OF ELEMENTARY ZOOLOGY

•substance they become pushed apart by it. In calcified
* cartilage the structure is the same as in hyaline, but
the ground substance is impregnated with salts of
hme. Connective tissue , like cartilage, has a ground
substance which stains readily with silver nitrate, but it is
.much softer and always contains fibres of two kinds—
white fibres, which have a wavy course, branch but do
not join, and are composed of extremely fine fibrils, and
yellow or elastic fibres , which have a straight course,
branch sharply and join to form a meshwork, and are
not composed of fibrils. The white fibres swell up and

become transparent
in acetic acid, the
elastic fibres do not.
When connective
tissue is boiled, the

d ground substance
yields gelatin ,
whereas cartilage
treated in the same
way gives a sub-
stance known as
chondrin. In the
ground substance
1 are a number of
irregular spaces
; occupied by cells
or connective-tissue
corpuscles , of which

Tig. 71. -A transverse section of a medul-
fated nerve of the frog, stained with
osmic acid and magnified.

■ar.y Artery; c.t., connective-tissue sheath or peri-
neurium funiculus or bundle of nerve fibres ;
v., vein.

•some are branched and often continuous by their branches,
while others are rounded and granular. Connective tissue
penetrates every part ot the body, holding together the softer
tissues and forming in the skin (pp. 34, 35) a continuous
■envelope known as the dermis, which is covered by the
-epithelial layer, known as the epidermis, already described
(P- 11 3)* Tendon is a modified form of connective tissue
in which the white fibres are very plentiful and run paral-
lel instead of forming a feltwork. The cells lie in rows
between the fibres. Fatty or adipose tissue (Fig. 80) is a
dorm of connective tissue in which the figures are scanty
and most of the/ cells are swollen into vesicles by the

THE FROG : HISTOLOGY , GERM CELLS , DEATH 121

presence of a droplet of fatty matter to which the proto-
plasm ot the cell forms a fine sheath with the nucleus on one
side. In the early stages of the growth of such a cell small

.A jc t v.f.

B-

p,con. p> e.x.

Fig. 72.- — Connective tissues of the frog.

A, Tendon; B, pigment cells in the skin, seen through the epi-
dermis.

c. Cells ; p.con., pigment cell with the pigment contracted ; p.ex.,
pigment cell with the pigment extending into the processes;
w.f.. white fibres.

droplets of fat are laid up in the protoplasm, and these
grow and run together till they fill nearly the whole cell.

Bone (Fig. 7) has a firm ground substance like cartilage,
but differs from the latter both in composition and in the

122

MANUAL OF ELEMENTARY ZOOLOGY

arrangement of its cells. The ground substance consists of
an organic basis impregnated with salts of lime — principally
the phosphate. When boiled it yields gelatin. It is
arranged in lamella separated by rows of minute spaces or
lacuna which contain the bone cells or bone corpuscles . The
lacunae are connected by fine canaliculi , through which
the cells are continuous by minute processes. The lamellae
are arranged in a concentric manner around spaces which
contain blood vessels. Some ot these spaces are large and

Fig. 73. — A transverse section of the spinal cord of man, taken
through the lumbar region, between nerves, with the pia mater
removed. Lettering as in Fig. 49.

filled with a tissue known as bone marrow , rich in fat cells.
When the layer of bone around the marrow cavities is thick,
it is traversed by smaller spaces, known as Haversian canals ,
in which lie minute blood vessels. The lymph from the
blood vessels permeates the bone through the canaliculi.

Blood is classed among the skeletal tissues on account of
the plentifulness of its fluid ground substance, although it
only acts as a supporting tissue when, under high pressure,
it renders an organ turgid. The fluid part of blood is
known as the plasma , the cells as blood corpuscles. They
are ol two kinds, red and white. Each red corpuscle is
a thin, biconvex, oval disc, yellow when solitary but
giving with its fellows a red colour, The colour is due to

THE FROG: HISTOLOGY , GERM CELLS, DEATH 123

a compound of a protein with an iron-bearing organic
substance. This compound which is called haemoglobin ,
belongs to a class of substances known as respiratory
pigments. It has the power of uniting with oxygen to
form a loose compound known as oxy haemoglobin, which
is formed in the respiratory organs where the pressure of
oxygen is high and breaks down, yielding its oxygen,
in the capillaries of the tissues,
where the oxygen pressure is low
(p. 76). Thus it serves as a carrier
of oxygen. The carbon dioxide
carried by the blood on the return
journey is held partly in simple
solution but mainly as bicarbonates
which dissociate in the chemical
conditions of the respiratory
organs. Haemoglobin is found in
the red corpuscles ot all back-
boned animals, but in man and
other mammals these corpuscles
are round biconcave discs without
nuclei. The white corpuscles or
leucocytes are colourless, smaller
and fewer than the red. They
have not, like the red, a definite
shape, but consist of very soft
undifferentiated protoplasm, which

has kept its power of contraction, and, when the corpuscle
lies against a solid surface, is constantly changing its
shape, putting forth in all directions irregular processes or
“ pseudopodia and as readily withdrawing them again.
By continually lengthening a pseudopodium and with-
drawing those on the opposite side they can flow along.
Movement of this kind is called amoeboid because it occurs
in a minute organism known as Amoeba , which we shall
presently study. In the blood stream the leucocytes take
on a rounded shape, so that they are easily bowled
along. The white corpuscles are of several kinds. Some
of them are of use to the organism by flowing round and
thus engulfing into their protoplasm harmful bacteria and
other parasitic micro-organisms, which they digest. An-

FiG, 74 — A human white-
blood corpuscle which
has ingested a number
of Micrococcus pyo-
genes aureus , the
common bacterium of
boils, etc. The micro
cocci are the small
dark spots ; the large
three-lobed structure
is the nucleus.— From
Haldane and Huxley.

!I24 MANUAL OF ELEMENTARY ZOOLOGY

•other kind removes the remains of dead tissues in a similar
manner. Corpuscles which thus devour objects are known
as phagocytes. Other leucocytes wage a chemical warfare
against the micro-organisms by secreting substances
{ antibodies ) which act against the latter in various
ways. Some of these substances {agglutinins) hamper
the bacteria by causing them to stick together or
^ agglutinate ’’ ; others {op sonins) facilitate the action

Fig. 75 —Inflamed mesentery of a frog, highly magnified.

a, Leucocytes ranged against the walls of a capillary ; b, leucocytes
migrating through the capillary walls ; c, escaped red cor-
puscles ; d, accumulation of leucocytes outside the capillaries.

— f rom Starling, after Adami.

of the phagocytes ; others {iysins) kill and break up
the bacteria ; others {antitoxins) neutralise the action
of the poisons, secreted by the bacteria, which are the
•cause of the harm which the latter do. Most of the
antibodies act only against a particular species of micro-
organisms. It they are successful they confer upon the
animal an immunity which may last for years, so that a
second attack of the disease caused by the micro-organism
in question will not occur. Inflammation is due to the
Hushing with blood of the part attacked, so that it becomes
swollen and red, and, in a warm-blooded animal, hot.
The white corpuscles then line up on the walls of the capib

THE FROG: HISTOLOGY , GERM CELLS , DEATH 125

laries, pass through them, and engage the enemy.
is lymph full of living and dead leucocytes and bacteria.
Leucocytes also carry substances, such as tat globules,,
from one place to another.

Fig. 76.— a portion of the bladder of a frog, stained and highly

magnified.

c.t.c ., Connective-tissue cell ; m.f, unstriped muscle fibres; ,»•*/»• . nuclei of the
epithelioid cells which cover and line the bladder.

The white corpuscles are semi-independent portions of
The Differ the organism living in the blood. Each of

entlation of them retains all the powers ot a whole
Ce,ls• organism. They are irritable, as may be seen

by their increased activity on warming, or by the effect
upon them of various drugs. 1 hey appear to be automatic,
for we can often trace their movements to no stimulus.
Their substance must undergo katabolism, tor _ they
expend energv, as we have seen, in contraction and in the
manufacture and secretion of various substances. The
fact that the stimulation of one part ot their surface causes-
other parts to act, as in changing the direction ot move-

126

MANUAL OF ELEMENTARY ZOOLOGY

ment shows the existence in them of conductivity. They
assimilate from the plasma nourishing matters to repair
t eir waste. They reproduce by fission, first the nucleus
and then the cytoplasm parting into two, and each half of
the cytopksm taking a half of the nucleus. Consisting as
it thus does of protoplasm which retains all the primary
powers of a living being, the white corpuscle shows us that
all these powers must be regarded as the birthright of all

protoplasm, and that their possession by
every organism is due to this fact, and
not to the presence of several kinds of
protoplasm. Yet it is important to
notice that the independence of the
leucocytes is only relative. They are
still wholly dependent for their nourish-
ment upon the body by which they were
formed, and their activity is directed
to the welfare of that body. With
the relative independence of the white
corpuscles, the condition of the other
cells of the body is in contrast. Each
of them consists of a portion of the
protoplasm of the organism in which
certain of its powers are highly de-
veloped, while others are degraded or
lost- It is probable that all protoplasm
retains irritability, either to stimuli
through a nerve or at least to changes
Fir 77. -A portion in the composition of the fluid that

fibre, magnified. t , Tt , ma^ be> too, that

automatism of a kind is widespread ;

r . mid disintegration and assimilation are

of course universal. But in most tissues the cells have
lost m the adult the power of reproduction, and certain
of the modes of appearance of the energy liberated by
disintegration are developed at the expense of others
Ihus in a nerve cell conductivity is highly developed and
contractility lost, while m a muscle fibre conductivity is rela-
tively low and contractility highly developed. In both these
tissues chemical manufacture and secretion for the benefit
of the rest of the body is at a low ebb, while in gland cells,

THE FROG: HISTOLOGY , GERM CELLS, DEATH 127

which are neither contractile nor conducting, it is highly
developed. In correspondence with these peculiarities cf
function are the peculiarities of form which we have noted.
That is to say, here as everywhere we find differentiation
and the division of physiological labour hand in hand.

The blood corpuscles die and are replaced, but not by multiplication
in the blood. The white corpuscles arise in adejioid or lymphoid tissue
which is a connective tissue
full of leucocytes and occurs
in patches known as lym-
phatic “ glands ” in various
parts of the body, especially
under the mucous membrane
of the alimentary canal and
along the lymph vessels, by
which they are conveyed
into the blood. The red
corpuscles of the frog prob-
ably arise in the spleen.

Those of mammals are
formed in a vascular tissue
known as red marrow which
occurs in the bones of these
animals.

When blood is shed
it clots , owing to the
precipitation in the
plasma of a protein
known as fibrin , in the
form of a meshwork of
fine fibrils which en-
tangles the corpuscles
and forms a firm mass.

Fibrin is formed by the union of the protein fibrinogen with
a small quantity of another substance thrombin , which,
under the influence of calcium, arises upon foreign surfaces
when the blood is shed. The liquid which remains after
the formation of the clot is known as serum. The effect of

Fig. 78. ~ Cartilage stained and mag-
nified, showing cells, some of which
are in pairs formed by the division
of a single cell, matrix, and the
newly secreted part of the matrix,
which forms capsules around the
cells.

clotting is to close wounds and thus prevent loss of blood.
The innumerable nuclei of the frog’s body have all
arisen by the division of one original nucleus,
that of the zygote formed by the union of the
ovum and spermatozoon (see p. 137). The
process of nuclear division by which the nuclei multiply

Me clear
Division.

MANUAL OF ELEMENTARY ZOOLOGY

128

is usually followed by a cell division in which each half of
the divided nucleus takes its own portion of the cytoplasm
which surrounded the parent nucleus, but in some cases,,
as in the division of nuclei for a striped muscle fibre,,
cell division does not take place, so that a ccenocyte arises.
Nuclear division is of two kinds. In a few cases, as in

71’. f

e.

Fig. 79. — Areolar connective tissue of the frog.
c., Cells ; e.f., elastic fibres ; w.f., white fibres.

some leucocytes, the process is quite simple. The nucleus
lengthens, then narrows in the middle so that it becomes
dumb-bell shaped, and finally breaks into two at the
narrow part. This is simple or amitotic division. In most
cases, however, a complicated process known as karyo-
kinesis or mitosis takes place. Outside the nucleus lies
a minute body known as the centrosome. When mitosis
is about to take place, the centrosome divides into two

THE FROG: HISTOLOGY , GERM CELLS, DEATH

1 29

parts which travel to opposite sides ot the nucleus. As
they separate, the protoplasm becomes arranged in a
radiating manner around each, forming a figure known
as the aster. Meanwhile the nuclear membrane is breaking-
up and the nucleus is undergoing certain internal changes
(Fig. 83). The linin with its contained chromatin granules
appears to form a coiled thread, the skein or spireme.
This, however, is made up of a number of portions, the
chromosomes , which
now shrink apart.

The same number of
them appears in every
nuclear division in the
body (but not, as we
shall see, in the germ
cells). In the frog
this number is twenty-
four. At this stage
there appears, stretch-
ing from one centro-
some to the other
across the site of the
nucleus, a gelatinous
spindle , divided by
more fluid “ threads ”
into strands diverging
from each centrosome
to the equator. The
proceedings up to this
point constitute the
prophase of mitosis.

Now the chromosomes become attached to the spindle in a
ring round its equator. The point of attachment in each
chromosome is known as the chromomere. At the same
time a split, of which there have already been indications,
divides each chromosome longitudinally into equal and
similar halves, known as chromatids. This is the meta-
phase. I he next stage {anaphase) consists in the passage
of the halves as daughter chromosomes along the threads
towards the poles of the spindle. There they arrange them-
selves in a radiating manner. Finally, in the telophase ,
Q

Fig. 80. —Part of one of the fat bodies of a
frog, compressed and magnified, show-
ing fat cells with fat globules in various
stages.

f-g., Fat globules; nu., nuclei.

130

MANUAL OF ELEMENTARY ZOOLOGY

each group thus formed constitute a

the chromosomes of
daughter nucleus
by passing through
a series of changes
which reverse
those by which the
mother nucleus
broke up. It will
be seen that the
result of this pro-
cess is an exact
halving of the
chromatin of the
mother nucleus be-
tween the daughter
nuclei.

The nuclei of the
ova and

Gameto-

genesis. sperma-

tozoa,
destined to form
by fusion a single
nucleus from which
the body nuclei arise, contain each only half the number of
chromosomes found in the body nuclei. This is the result of

certain peculiarities
in the cell divisions
by which the gametes
arise. The formation
of gametes is known
as gametogenesis.
That of spermatozoa
is spermatogenesis ,
that of the ovum is
oogenesis. Gameto-
genesis ends by
maturation — two suc-
cessive divisions of a
cell, known as the
gametocyte , derived

Fig. 8i.

Blood of a frog, highly magnified.

A, Fresh ; B, stained.
leu., Leucocytes ; r.c., red corpuscles.

Fig 82. --Human blood, highly magnified
leu. , Leucocytes; r.c., red corpuscles.

from the germinal

THE FROG : HISTOLOGY , GERM CELLS , DEATH 131

epithelium. The gametocytes of spermatogenesis (Fig. 55 1)
are known as spermatocytes ; those which give rise to ova
are oocytes.

The first division of maturation is known as the meiotic
division and the procedure in it as meiosis. It differs
from ordinary mitosis in that by it the number of chromo-
somes is halved , and on that account it is said to be a
reducing division. The reduction is due to the fact that,

Fig. 83. — Kayokinesis — After Flemming.

1. Spireme stage of nucleus ; c.c., centrosome.

2. Longitudinal splitting of chromosomes, and arrangement of them on the

equator of the spindle ( Metaphase ).

3. Separation of halves of chromosomes (Early anaphase).

4. Recession of daughter chromosomes from equator of cell.

5. Nuclear spindle, with chromosomes at each pole (Late anaphase ).

h. Retrogression of nucleus ( Telophase ) and cell division.

instead of dividing to provide one daughter chromo-
some for each daughter nucleus, each chromosome goes
undivided into one of the daughter cells. The process is
complicated by the fact that the true chromosomes come
together in pairs in the gametocyte before the meiotic
division, so that the nucleus appears already to contain
half the normal number of chromosomes, but these are
really double chromosomes. It is not any two chromo-
somes which become partners in a double chromosome,

132

MANUAL OF ELEMENTARY ZOOLOGY

but two which (as is known from facts which we shall
note later) are similar through not identical -in con-
stitution and often have visible common features which
distinguish them from the others. The members of such
a pair are known as homologous chromosomes . In fact,
each cell of the body has a double set of chromosomes,
and it is corresponding members of the two sets that

Fig. 84. — Diagrams of stages in meiosis. Two chromosome-pairs
are shown : the members of one are short and form one ehiasma,
those of the other are longer and form two ehiasmata (ch).

1. leptotene ; 2. zygotene ; 3. four-strand pachytene ;

4. diplotene ; 5. diakinesis ; 6. metaphase.

come together at meiosis. The two chromosomes of each
double chromosome part in the meiotic division, so that
the daughter nuclei have each half the normal number of
chromosomes, but these are now of the ordinary single
kind. While they were together, however, the chromatids
which had by then appeared in each of the chromosomes
of a pair exchanged lengths of themselves with the
chromatids of the other chromosome, so that those which
part are not identically these which came together, but
each is a combination of parts of the former pair. The

THE FROG: HISTOLOGY , GERM CELLS, DEATH 133

lengths exchanged are corresponding portions, so that no
portion is duplicated in either chromosome. This process
is known as crossing over (Fig. 85).

Meiosis is a complicated process which varies in detail, and descrip-
tions ot it are the harder to follow on account of differences m the
application of the terms which have been applied to its phases. In
typical instances it proceeds by the following stages :

1. Leptotene stage. The chromosomes are slender, not split
longitudinally, and tangled much as in the spiieme.

1.

A homologous pair of chromosomes ; m, the chromosome of maternal,
h, that of paternal origin ; 2, pairing ; 3, splitting into chromatids ;
4 chiasma formation ; 5, 6, separation (chakmesis) with adherence at
one end • 7 parting of chromosomes at anaphase ; a, point at which
chromosome becomes attached to spindle ( chromotnere ) . The shorten-
ing of the chromosomes and the temporary obliteration in the pro-
metaphase of the split between the chromatids are not shown.

2. Z,ygotene stage. The chromosomes, which are shorter thicker,
and less tangled, are coming together in pairs whose members are
closely applied (but not fused), the union generally beginning at one
or both ends. This union is pairing (in earlier accounts synapsis, , or
syndesis , or conjugation . The chromosomes tend to crowd together

towards one side of the nucleus (synizesis) .

3. Pachytene stage. The chromosomes are still shorter and thicker

and the pairing has been completed.

(a) Tuo-strand pachytene. The chromosomes remain unspht.

134

MANUAL OF ELEMENTARY ZOOLOGY

(b) Four-strand pachytene. Each chromosome is split longi-
tudinally into two chromatids. There now happens an
event which is not revealed till the next stage : in places
two adjacent chromatids belonging to different chromo-
somes of a homologous pair break and unite reciprocally
by their broken ends.

In the late two-strand and the four-strand pachytene stages the
chromosomes of a pair are often twisted round one
another. This is known as the strepsitene condition.

4. Diplotene stage. The chromosomes continue to shorten and
thicken and the two which joined to form each double chromosome
now separate again, perhaps because the attractive force which held
them together is now used up in holding together the chromatids in
each of them. They do not, however, become completely separated
but remain attached at certain points. These are the points at which
the breaking and reciprocal union of chromatids has taken place.
As the chromosomes separate, the chromatids at each of these points
form an X which is known as a chiasma. Presently the chromosomes
draw apart at the chiasmata too, portions of each chromatid being
thus drawn into the chromosome to which it did not at first belong.
Usually the chromosomes of a pair still adhere at both ends, forming
a ring, or at one end where they form a cross. In the late diplotene
stage or diakinesis the chromosomes, now very short and thick,
disperse through the nucleus.

5. Prometaphase ( stage of gemini). Further contraction forms
ring-shaped or cross-shaped double chromosomes. The splits
between chromatids are temporarily obliterated in the chromosomes.

6. Metaphase. Each double chromosome has two chromomeres,
one for each of its constituent chromosomes.

7. Anaphase. The chromomeres move to opposite poles of the
spindle, each drawing with it two (usually much shortened) chroma-
tids. Chromomeres of the original (paternal and maternal) sets
go indifferently to either pole so that the original sets do not segregate.

The second or post-meiotic division follows the first
without a resting stage. It is an ordinary division, in
which each chromosome splits into two to give rise to
one daughter chromosome in each daughter nucleus, so
that the daughter nuclei still have each half the normal
number. At fertilisation the zygote has therefore the
normal number, which it imparts to the body cells during
the mitoses by which it forms them.

Spermatogenesis takes place in the testis (cf. Fig. 551).
Oogenesis begins in the ovary and finishes after fertilisation.

The actual course of maturation differs greatly in the
two cases. In spermatogenesis the cytoplasm is equally
divided and each of the four products (. spermatids ) be-
comes a spermatozoon, so that from each spermatocyte four

THE FROG: HISTOLOGY, GERM CELLS, DEATH 135

spermatozoa arise. In oogenesis each division is unequal
\t the first division there are formed a large cell and
a small one, the latter containing very little cytoplasm

Spermatogenesis Oogenesis

Fig, 86. —A diagram of gametogenesis and fertmsation.-

and being known as the first polar body . At the second
division, while the first polar body forms two very small
cells,1 the large cell forms again a large and a smalt
1 In many animals the first polar body does not divide.

136 MANUAL OF ELEMENTARY ZOOLOGY

product. I he large product is an ovum ; the small is
called the second polar body. Thus, instead of four ova
the oocyte forms one ovum and three vestigial cells which
come to nothing. The formation of the polar bodies is
known as the maturation of the ovum.

The events just described have important consequences :
(1) the fact that, so far as chromosomes are concerned
the processes are the same for both kinds of gametes
makes the nuclei of the ovum and spermatozoon contribute
equally to the zygote nucleus ; (2) owing to what happens
at the reduction division, the zygote does not get all the
chromosomes of a cell of each of its parents, but only one
hall their number. Thus at each fertilisation the number
ot chromosomes does not increase by doubling as otherwise
would happen ; (3) while this number remains constant,
it is made up by a new combination, consisting of an
equal set from each parent ; this is the source of the two
homologous sets mentioned above ; (4) since these sets are
provided (at the reduction division) not by the splitting of
chromosomes (which would give identical sets) but by the
parting ot whole chromosomes, any difference that may
exist between the chromosomes of a pair will cause the
germs to which they are transferred to be unlike in that
respect ; (5) crossings over cause the chromatids of a
chromosome to differ, but the second division separates
them, and each gives two like chromatids in a zygote if it
enter one. In the upshot, the chromosome material of a

zygote has not increased, but is a new entity in two ways

because it has come from two parents, and because the
contribution from each parent came (by crossing over)
partly from each of two sets of chromosomes.

Only one spermatozoon ever unites with any ovum.
Fertilisation. ^ slime around the eggs is swelling up

and setting to a jelly in the water, the spermato-
zoa which have been shed over it by the male (p. 81) pass
through it, swimming by means of their tails. They are
far more numerous than the ova and most of them perish
but one succeeds in entering each egg. Thus a zygote
known as the oosperm ox fertilised ovum comes into being.
After fertilisation the ovum shrinks from its vitelline mem-
brane. I he cytoplasm of the spermatozoon disappears in

THE FROG : HISTOLOGY , GERM CELLS , DEATH 137

that of the ovum, but the nucleus passes onward and comes
to lie side by side with that of the egg. The two nuclei are
known as the male and female pronuclei. Meanwhile there
has arisen from the neck ol the spermatozoon a centrosome,
around which is formed an aster. As the nuclei approach
one another this divides and forms a spindle. The pro-
nuclei break up each into twelve chromosomes, which lie
at the equator of the spindle. Thus the normal number
of twentv-four chromosomes is restored ; these lie in two
groups corresponding ^

to the two pronuclei.

Now the chromosomes
split in the ordinary
way and the halves pass
to opposite poles ol the
spindle, where they form
ordinary nuclei. The
cytoplasm of the egg
meanwhile divides into
two cells, known as the
first two blastomeres .

This is the first of the
series of divisions known
as the cleavage or seg-
mentation of the ovum ,
by which the cells of
the embryo are formed
(see p. 616). At these
divisions each cell pro-
duced receives, by the
mechanism of ordinary

mitosis, a complete set of chromosomes from each parent
of the original zygote.

The frog which is developed from the fertilised ovum

will, like any other animal, inherit the like-
Inheritance. Qf ks parents . large frogs will have large

offspring, and so forth. d his, of course, because the
embryo is physically continuous with each parent through
one of the germs. The question suggests itself : is any
constituent of the germ specially responsible for heredity ?
Since the nucleus is known to exercise a directive influence

Fig. 87. — Sections of the egg of a frog
during maturation and fertilisation.—
Semi-diagrammatic.

1, 2, 3, Successive stages.

nu., Nucleus of ovum, still in the pigmented
region, but already shrunken and lying in a
clear space formed by fluid extruded when
the nuclear membrane was dissolved ; p.b.i,
p.b.2, first and second polar bodies; pig., pig-
mented protoplasm ; $, female pronucleus ;

g, male pronucleus ; the track along which
this has entered is shown by pigment.

138 manual of elementary zoology

over many of the activities of the cytoplasm, and again
since the spermatozoon contributes a full quota of nucleo-
plasm but hardly any cytoplasm to the embryo, it is
natural to conclude that the principal though perhaps not
the only agent of heredity is the nucleus. This probability
gives a peculiar interest to the fact, which we have already
noted, that, as regards the nucleus, ovum and spermato-
zoon make equal contributions to the embryo. Thereby,
it would appear, the parents make equal contributions to
most, if not to all, of the inheritance of the offspring.
Because it is the material in the chromosomes that is
meticulously shared out between the offspring, and because
in certain cases — most clearly in that of sex (see p. 701) —
particular characters of the offspring have been found to
follow particular chromosomes, it is held that this contribu-
tion is made by means of the chromosomes. As to the way
in which such contributions realise themselves when they
are of opposite tendency (that is, when two homologous
chromosomes differ in some respect)— how they some-
times blend and sometimes one dominates over the other
so that the offspring “ takes after ” one parent — we shall
have something to say in later chapters (pp. 598 and 697).
The shuffling of chromosome material which takes place
in the maturation and union of gametes ensures that no
two individuals have the same chromosome constitution
and thus promotes variety. We shall also see the im-
portance of this later on.

It is said that frogs have been known to live twelve
Death years. Sooner or later, however, they, like

ourselves, must die. However successful the
individual may be in avoiding enemies and accidents, he
cannot escape that gradual slowing of the working of the
bodily machine which in the long-run brings it to a stand-
still. The course of metabolism is in some way limited,
so that in the most perfect conditions natural death would
eventuallv result. The nature of this limitation is not
understood. The fact that the germs are not subject to it,
but produce new individuals with a fresh lease of life, shows
that it is not inherent in all protoplasm, but belongs only
to that of the cells which constitute the bulk of the body.
This it would seem to affect in all the tissues, so that,

THE FROG : HISTOLOGY , GERM CELLS , DEATH 139

were the body not the complicated machine that it is,
death might come as a gradual loss of power in all parts
alike. As it is, however, the end is always more or less
premature as regards some tissues, being brought about by
the breaking down of one of the main parts of the machine,
as the brain, or lungs, or heart, though in the long-run any
other part than the heart brings about general death
through its effect upon that organ, whereby the rest of
the body is deprived of fresh blood. It may be that this
breakdown is due to the protoplasm of the body ceils
producing substances which encumber them or act as
slow poisons, and that these in course of tune accumulate
beyond the power of the body to destroy or excrete them.
Or it may be that they have not the power of renewing
parts in them which wear out. In any case, so far as our
present knowledge goes, death in all the higher animals is
inevitable.

CHAPTER V!

AMCEBA

Amceba proteus is a little organism found in the mud
and on weeds in freshwater ponds. A large
Features. specimen is just visible to the naked eye as a
minute, irregular, whitish speck. Under the
microscope (with transmitted light) this is seen to be a mass
of translucent slime, greyish in colour owing to the presence
of numerous small, dark granules. The outer layer is
clear and transparent owing to the absence of the granules.
This layer is called the ectoplasm , the granular inner
part being the endoplasm. In the endoplasm are usually
to be seen the remains of other little organisms, especi-
ally minute plants, which form the food of Amoeba. At
one spot is a round space filled with a clear fluid, which
grows gradually larger and then suddenly disappears, owing
to a contraction of the protoplasm around it causing it to
burst out and discharge its contents into the surrounding
water. It then gradually re-forms in the same portion of
protoplasm as before. This space is called the contractile
vacuole . There are usually other small vacuoles which are not
contractile. With care there may also be seen in the living
specimen a lens-shaped body of moderate size which is
somewhat denser than the rest of the protoplasm. This is
the nucleus. If the animal be killed and stained with
carmine or any of a number of other dyes, the nucleus
takes up the stain more deeply than the cytoplasm. The
irregular shape of the body is constantly changing, owing
to the outgrowth of new processes or pseudopodia and the
withdrawal of old ones.

1 40

AMLEBA

141

The formation of a pseudopodium begins with a slight
outflowing of the ectoplasm, into which the
Movements. endoplasm presently flows. The projection

continues to grow by the flow of more protoplasm into it
for a varying time, and locomotion is brought about by
the persistent lengthening of one pseudopodium till the
bulk of the body has been transferred into it. During
this time it is throwing out subsidiary pseudopodia
in various directions. Before very long, however, the
main flow is directed into one of these and the animal

Fio. —Amoeba proteus , highly magnified.

e.&„ Contractile vacuole ; ec., ectoplasm ; en., endoplasm ; /.p food particlis ;

nu. , nucleus; ps.t pseudopodia.

moves in another direction, the stream in the older pseudo
podia setting backward into the body until they disappear.
The flow of the endoplasm is always swifter in the middle of
a pseudopodium than at the sides. It will be seen that \ve
have here an example of contraction , as the word is used in
Biology, the shape of the mass of protoplasm being changed
by the transference of material, but the size . remaining
the same. The throwing out of a pseudopodium is not
brought about merely by a flowing of the protoplasm, but
takes place in the following manner. 1 he ectoplasm and
the outer part of the endoplasm together form a firm coat,
the plasmagel, around the fluid inner endoplasm or

142

MANUAL OF ELEMENTARY ZOOLOGY

plasmasol. Where a pseudopodium is to be thrust out the
plasmagel softens, and the contraction of the rest of that
layer then presses the plasmasol towards this spot, which
bulges. As the bulge grows, a covering of plasmagel for
its flanks is provided by conversion of plasmasol. Over the
ectoplasm is a very thin elastic pellicle, th e plasmalewi may

Fig. 89. —Successive changes in shape of an individual of Amoeba
proteus , drawn at intervals of two minutes.

which is sticky and therefore adheres to the ground where it
is in contact, so that the effect of the forward thrusting of
the protoplasm within is to roll it along, as an india-rubber
bag filled with water may be rolled over a surface, and thus
the animal travels in the direction of the thrust. Pseudo-
podia which do not touch the ground merely protrude
without causing locomotion, but the creature may place
their tips upon the ground and thus walk upon them.

A MCE BA

143

When it is floating freely it puts out slender, finger-like
pseudopodia and appears to be searching with them for
foothold. During the movements the contents of the
endoplasm — nucleus, food particles, etc. — are carried
about freely from place to place in the body, but the
contractile vacuole adheres to the inner surface of the
ectoplasm and moves with it. The constant changes of
position of internal bodies is one of the arguments which
support the foam theory of the structure of protoplasm
(see p. 104) against theories which demand the existence
in it of a meshwork of fine threads, and an examination
under high powers of the
microscope confirms this
by revealing appearances
similar to those found
in certain artificially
made foams.1 Arti-
ficial foams can even
be caused to carry out
movements which in
their general features
resemble those of
Amoeba. It should be
noted, however, that the
special features of the
•contractions of Amoeba
are not found in them.

The pellicle which covers the surface of Amoeba is believed

to be rich in fatty substances.

Amoeba feeds on small organisms, which it ingests by
surrounding them with outgrowths of its
Nutrition. protoplasm and so engulfing them. The
space in the body which the prey comes to fill would
thus be lined with ectoplasm, but the ectoplasm here
becomes absorbed into the surrounding endoplasm, so
that it is clear that there is no essential difference between
the materials which compose these layers. There is then
secreted around the lood particle a layer ol water con-
taining substances which kill it and digest its nourishing

1 Such a foam may be made by mixing together rancid oil and salt
.and placing little droplets of the mixture in watei.

Fig, 90. — Amoeba proteus in the act
of ingesting /, a small vegetable
organism which is being swallowed.

144

MANUAL OF ELEMENTARY ZOOLOGY

part. The reaction of this fluid is acid at first, but later
becomes neutral or slightly alkaline. The space containing
the digestive juice is known as a food vacuole. The chief
tood of Amoeba is protein. It is said also to digest carbo-
hydrates, but not fat. The dissolved substances are
incorporated, and the undigested parts are egested by the
simple process of being left behind as the animal flows
along.

The protoplasm of Amoeba is irritable, automatic, and
. conductive. Its irritability is not, as in higher

irritability ,

Automatism, animals, specially developed m sense organs,

Conductivity. !3Ut that th*s Property exists in it is shown
in various ways. It Amoeba be stimulated by
slight contact or by meeting very dilute solutions of
various chemical substances it will form a pseudopodium
on the side towards the stimulus. If it be pricked with
the end of a fine thread of glass, or come into contact with
stronger solutions of chemical substances, it will draw
back and flow away. In this case the formation of a
pseudopodium in a region of the body other than that
which has been stimulated shows the presence of con-
ductivity. Again, it does not swallow every particle it
comes across, but chooses those that either contain
nourishing substances or are in motion (in which case
they are probably alive and therefore fit for food). By
an unkind deception of this “ sporting instinct,” it
may be induced to capture and swallow moving particles of
glass. Its mode of seizing food is not fixed, but adjusted
with an uncanny appearance of intelligence to the nature
and behaviour of the prey of the moment, which it dogs
with perseverance and resourceful changes of method.
It will move away from strong light, but does not appear
to perceive a particle of food better in the light than in
the dark. All this shows that it receives from foreign
bodies various stimuli, and discriminates between them.
In contrast to these instances, many of its actions cannot
be traced to any stimulus, and must therefore be classed
as automatic in the sense in which we have used that
word. In much of its activity it appears to be exploring
its surroundings and to continue on a course until it
receives some stimulus which repels it, but sometimes, as

A MCE BA

145

Excretion and
Respiration.

Depression.

in capturing food, it appears to be attracted in the direction
from which a stimulus comes.

The contractile vacuole is probably an organ for the
regulation of the water content of the proto-
plasm. Water must enter all over the surface
of the organism and more is produced during
metabolism. The excess which results is collected into the
vacuole. The water expelled must take with it dissolved
carbon dioxide, and thus the contractile vacuole aids
respiration. Possibly it also removes excreta. At the
same time it seems likely that the whole surface of the
body serves to some extent
both for respiration and for
excretion.

Unfavourable conditions
of life may bring
about a disease
known as depression , in
which the nucleus of the
Amoeba is enlarged and the
various functions become
deranged. This disease, how-
ever, is more familiar and has
been more closely investi-
gated in some other minute
organisms, as, for instance,
in Paramecium (p. 171).

In certain circumstances
Amoeba withdraws its pseudo-
podia and becomes a rounded mass which secretes about
itself a tough case or cyst A In this it lies dor-
mant and can survive the drying or freezing of
the pond in which it lives or be transferred in mud to other
ponds. We have here an instance of a widespread pheno-
menon known as suspended vitality , which is found, for
instance, in seeds and in frozen tissues (see p. 107). The
exact condition of the protoplasm in such cases is a mystery,
but no vital processes can be detected, and it has been

1 It is doubtful whether the resting cyst of A. proteus has been seen.
Both resting cysts and cysts for spore formation (see p. 147) are known
in other kinds of Amoeba.

10

Fig. 91- —A diagram of the fission
of Amoeba. The dark spots
represent nuclei.

Encystment.

146

MANUAL OF ELEMENTARY ZOOLOGY

shown by experiments on seeds that, if they be kept
perfectly dry, not even respiration takes place. We must

conclude that life, regarded

as a process, has slowed
down and, at least in some
cases, ceased, but that the
protoplasm retains the power
of resuming it in certain
circumstances. At death, on
the other hand, the proto-
plasm passes into a condition
in which it will indeed remain
intact in suitable circum-
stances (as when it is frozen)
but has lost the power of
resuming life.

Amoeba reproduces by the
process known
as binary fission.

Reproduction.

Fig. 02. — Multiple fission of
an Amoeba. — After Seheel.

A, Amoeba encysted ; P, section of a
cyst in which numerous nuclei have
been formed, more highly magnified ;
C, surface view of a ripe cyst in
which the spores are beginning to
separate and the cyst wall to break
up ; D, a single spore highly magnified.

in which first the nucleus
and then the cytoplasm parts
asunder into two halves,
each of which appears, at
all events, to differ from the
parent in nothing but size.
In some fissions of Amoeba
the division of the nucleus
is amitotic, but at other
times there is a peculiar
kind of mitosis in which the
place of centrosomes is taken
bv a mass of clear protoplasm
at each end of the nucleus.
These masses are known as
pole plates and arise within
the nuclear membrane,
which does not break up
mitosis.1 After the division

during division as in ordinary
of the nucleus the cytoplasm flows apart into two bodies,

1 Not all the chromatin takes part in the mitosis. The inner portion
alone does so : the outer portion is halved independently of the spindle.

AMCEBA

147

each of which contains one of the daughter nuclei. The
new bodies are at first connected by a bridge of protoplasm,
but this becomes narrower until it breaks through and
two new individuals come into being.1 Another kind of
fission, known as multiple fission or spore formation , takes
place at times. Its details differ with the kind of Amoeba.
In one kind it has been described as follows. The animal
■encysted and its nucleus divided amitotically till a very
large number (some 600) of small nuclei had been formed.
These passed to the surface of the cytoplasm, which
gathered into a little mass around each of them. The
■cyst wall was now dissolved and the little individuals or
spores escaped as small Amcebce with fine, pointed pseudo-
podia unlike the blunt processes of the adult, a residual
mass of unused cytoplasm being left behind. The young
forms grew and became transformed into adults. In
Amoeba proteus spores are formed without encystment.

Syngamy has not yet been proved to occur in Amoeba
proteus. The animal does, however, occa-
wuitjnuc|eate sionally undergo a process known as plas-
togamy , in which the cytoplasm of several
individuals fuses, forming a single mass which contains
several nuclei. Such a mass is known as a plasmodium.
Quite another kind of multinucleate body is found in
certain species of Amcebce and in the Amoeba- like animals
known as Pelomyxa , where two or more nuclei are formed
by the division of a single nucleus. I hese may be com-
pared with coenocytes.2 * * *

We have studied Amoeba as an example of extreme
simplicity in organisation. That is not . to
Surroundings*8 say that it 1S primitive in the sense in which
that term is used by zoologists ; it is quite
unlikely that this creature is a survivor from among
the earliest living beings. Indeed it is more probable
that if we could trace back the descent of the Amcebce
we should come to ancestors not unlike the organisms

1 Binary fission takes about an hour.

2 Groups ot similar, unseparated energids are known as syncytia.

They may be plasmodia , formed by the union ot tree energids, 01

sympiasts, formed by the division of the nucleus of a single energid.

A symplast may be a ccenocyte , or the whole body ot an organism.

1 4S

MANUAL OF ELEMENTARY ZOOLOGY

which will be described in the next chapter. But Amoeba
does contrive to carry on its life with less apparatus than
almost any other creature, possessing as it does no ob-
vious permanent organs except the cytoplasm, nucleus,
and contractile vacuole and besides these only the
temporary organs known as plasmalemma, plasmasol,
plasmagel, and pseudopodia. Certain reflections arise
from this fact. To begin with, if Amoeba can live success-
fully with such a simple organisation, why should there
be organisms such as the frog — or man — which use an
outfit so much more complicated ? The answer is, of
course, that Amoeba occupies but a narrow niche in its
environment. The world offers many possibilities of
which it fails to take advantage. The life of the frog is a
much greater achievement. Life, as we have seen,
consists in the making of adjustments in the living being
which enable it to survive and prosper amid the vicissi-
tudes of its environment. The smaller the range ol
adjustments it can make the smaller will be the range of
variation in its surroundings which it can survive — the
more limited will be its existence. The capacity of the
frog for adjustment to its surroundings is enormously
greater than that of Amoeba. It can live out ot water,
can feed on relatively large and active organisms, can
readily perceive enemies and swiftly and skilfully evade
them, can find a mate and carry out a form of repro-
duction which ensures variety in its offspring. There is
no need to show how much more command over the
forces of nature man has than the frog. The degree of
complexity of organisation in any organism is the degree
of variety in circumstances to which it can adjust itself — -
the degree of mastery that it has over the environment.
Higher and lower organisms exist side by side. This is
because they are not in competition. The higher organ-
isms avail themselves of circumstances (such as food and
facilities for respiration) which are beyond the powers of
the lower. When two organisms are indeed in competition
the better equipped will oust the other. This, as we shall
see when we come to study evolution, has very important
consequences in moulding the organic population of the
globe.

A MCE BA

H9

Nevertheless our survey of the life and structure of
' Amxba has shown us that it must be regarded

a c*nfSa as an organism in no way interior to the frog

in its fundamental powers. It is irrit-
able and automatic, undergoes kata-
bolism, contracts, conducts, does
chemical work, secretes and excretes,
respires, incorporates tood, an
reproduces.

In many points of structure ana

behaviour Amoeba resembles closely

the white corpuscles ot the trog and

more distantly the other cells of the

frog’s body. It is, in short, like them

a self-contained mass ot protoplasm

with a nucleus — an isolated energid.

- 1 • ^ Unp Koon nQiifll tO

oA

Vr A

witn a nucleus — an iov^v— - u

For this reason it has been usua o ^ _A diagram of
regard it as a cell, and to call it a the reiation of germ
unicellular organism, and a view is
widely held, on which the body ot
such an animal as the frog is said to be
a colony of units, each comparable to
a single Amoeba , specialised for co-
operation with the other cells ot t e
body. A nerve cell, lor instance, has
the function of conduction high y
developed but has lost those of secre-
tion and contraction. But the facts
maybe interpreted in another manner.

Amoeba is a complete and independent
organism comparable with the whole
body of the frog. Its small size
enables all the functions which the
nucleoplasm performs to be carried
out by a single nucleus In the
frog the size of the body makes
necessary a large number of scp- t Qf tpe

arate nuclei, and around each of thes : ^ an

energiddp Isolated^ Such L "energid isolated

withm the body is called a cell. Now the energids which

the relation of germ
and body substance
in the frog. The
dark circles represent
germs, the light cir-
cles body cells. The
germ gives rise in
each generation to
numerous body cells
which remain to-
gether and eventu-
ally die, and a. so to
germs (of which only
one is shown in each
generation). The
germs leave the body
and give rise each to
a new group of body
cells and new germs
Thus the germ sub-
stance is immortal,
the body substance
mortal.

MANUAL OF ELEMENTARY ZOOLOGY

I vO

art* thus isolated as cells have not all the properties pos-
sessed by the body as a whole, but they have special
qualities according to the functions of the part of the body
m which they lie. On this interpretation a cell is a portion
of the body of a whole organism which is specialised for
the performance of particular functions rather than a whole
organism which co-operates with other such organisms to
form a body of a higher grade. Amoeba is not a cell, but
an organism which is not divided into cells (non- cellular).
It is this view of Amoeba that we shall adopt.

1 he difference between Amoeba and the frog may be

Immortality *tate<^ }n another way. Viewed broadly, the

of Amoeba. formation of a germ by the frog is a separation

of the body into two portions, one small — the
germ— and another large, in which the individuality of the
parent is continued, f he parent consists mainly of energids-
which are specialised or differentiated, as nerve cells
gland cells, muscle cells, and so forth, for the performance
of certain functions in the body, and are correspondingly
unable to produce energids of other kinds. The germ is an
energid which is not thus specialised, though of course it
has an organisation of its own. Now the energids of the
parent body are mortal : that is to say, sooner or later they
undergo natural death. But the germ is in a sense
immortal : that is to say, unless it be devoured, or starved,
°r poisoned, or fail to find a mate, or meet with some other
fatality, it will not die a natural death, but in giving rise to
a new adult organism gives rise also to another generation of
germs, in which it continues its existence within the adult
organism until the latter in turn sets free germs. The
difference between Amoeba and animals like it on the one
hand, and higher animals like the frog on the other, lies in
the fact that in the former there are no bodv-cells, but the
whole body has the immortality of a germ. ' The fission of
Amoeba is a separation of the body into two similar products,
neither of which can be said, in virtue either of size or of
mortality, to represent the parent. There are two offsprin°r.
but the parent has disappeared.

In a later chapter (p. 179) there will be found descriptions
of organisms known as Entamcebce , closely related to>
Amoeba, which are parasitic within the body of man.

CHAPTER VII

FLAGELLATA

Water in which organic matter is decaying always contains
numerous small organisms of various kinds.
Poiytoma. Among these, when decomposition is well ad-
vanced, there can be found with the aid of the
microscope minute, colourless organisms of a species known
as Polytoma uvella , which feed by absorbing from the water
through the surface of their bodies substances in solution
derived from the decaying matter. The body of a Polytoma
is an egg-shaped mass of protoplasm without any internal
skeleton. A pair of long protoplasmic lashes or flagella
project from one end ; by a backward lashing of these it
swims with a somewhat jerky course, the end at which
the flagella are placed being forward (Fig. 95, A, B). The
permanent shape of the body is due to a thin cuticle ;
that is, not to a surface layer of the protoplasm, but to a
protective covering formed by secretion. It is pierced
by two pores for the flagella. Two contractile vacuoles
lie close behind the flagella and contract alternately.
There is one nucleus, placed somewhat behind the middle,
and there is sometimes a spot of red pigment situated in
the front part of the body. The hinder region contains
numerous starch granules. These must be formed by the
protoplasm from substances absorbed in the food : they
serve as a reserve of nutriment, and are used up during
starvation. Their presence is interesting, for starch, though
it is common in plants, is rare in the protoplasm of
animals, which, if they store carbohydrates, usually do so
in the form of glycogen. Together with the spot of red
pigment — which is an organ that enables small, motile,
green plants to find the sunlight which is necessary to their

152

MANUAL OF ELEMENTARY ZOOLOGY

mode of nutrition (p. 25) — the starch granules betray the
fact that Polytoma , though it has lost its chlorophyll, is at
least as much a plant as an animal. It is, in fact, a colour-
less Chlamydomonas (p. 28). Polytoma can encyst, and
in the encysted state is carried about in dust, etc., to
germinate in favourable circumstances elsewhere.

Reproduction is usually brought about by a process known

Reproduction as rePeaie^ fission , in which binary fission is re-
peated so as to form four daughters before the
young separate, but sometimes there are only two off-
spring. Fission takes place within the cuticle, this being

carried about during
the process by the action
of the flagella, which
remain attached to one
of the daughters. The
nucleus divides by a
kind of mitosis. The
first division is nearly
transverse, the second
at right angles to
it. The flagella are
then withdrawn, each
daughter forms twm
small flagella, and the
cuticle of the parent is
dissolved. At intervals of a few days syngamy takes
place. Two ordinary individuals come together and fuse,
their nuclei joining and their cytoplasm melting into one
mass, wrhich then encysts. After a resting period the
zygote divides by repeated fission into eight, each of the
daughters grows two flagella, and the cyst is dissolved. In
regard to this process we must notice (1) that syngamy
can occur at any time in the life of the individual, and does
not take place between special germs which cannot develop
without it : in the Irog, on the other hand, syngamy is
obviously impossible in the adult and can only take place
between the little germs, before they develop the rest of
the body ; (2) that the gametes are alike, and not, as in
the frog, ol two kinds, a passive kind, wThich bears the
bulk of the cytoplasm, and an active kind, by which is

c.v., Contractile vacuole ; cu., cuticle ;
nucleus ; s.g., starch grains.

FLAGELLA TA

J53

carried out the locomotion which the process involves.
-Both gametes m Polytoma are fairly well supplied with
cytoplasm and both are motile. Only when one is

I

D

*

o.

F!G. 95. — Movements of Flagella and Cilia.

A.

B.

C.

D.

The effective stroke in a flagellate which is swimming by lashing the water

tsry whi?.,r n^uum "t„ ,ts

Vote that the flagellum is stiff during the effective stroke and limp during
the recovery, so that in the latter it is bent and does less work. The
arrow shows the direction in which the creature moves. For simplicitv
the beat is represented as taking place in one plane : actually it is

fonvardsPwith;a sY-SUCh 3 b®at 3 Single flaSe]lum can draw the organism

A flagellate drawing itself forward by undulation of its flagellum. Waves
pass from tip to base of the flagellum, and their backward pressure on the
water moves the organism forwards, as in the swimming of a fish (see

A row of cilia contracting successively (in metachronalrhvthm) . e. Effective
stroke ; r., recovery ; l., direction in which liquid overlying an epithelium
is moved ; o., direction in which an organism is moved, by a covering of
cilia. Like the flagellum in A and B, the cilia are more flexible in the
recovery than in the effective stroke. The effective stroke is also usually
the more rapid of the two. y

older than the other is there sometimes a difference in
size.

In Chlamydomonas (Fig. io) syngamy takes place, not,
as might seem possible, between ordinary individuals,
but between special small forms which arise by repeated

154

MANUAL OF ELEMENTARY ZOOLOGY

fission of the ordinary forms. These ‘special gametes,
varieties of however, are like the ordinary individuals
Syngamy. in all but size. In some species of Chlamy-
domonas they are themselves of two sizes, which con-
jugate large with small, so that,
as sometimes in Poly tom a,
there is a difference in size,
though not in any other respect,
between the gametes. The
syngamy of two like gametes,
whether they be ordinary or
special individuals, is known as
isogamy ; syngamy of unlike
gametes is anisogamy ; if, as
in the frog, they differ not only
in size but also in that the
larger is passive and the smaller
active, the process is known as
oogamy.

Euglena viridis , often so
common in puddles
as to give them a
green colour, is a flagellate
organism of a rather higher
grade than C hlamy domonas or
Polytoma. It is a minute,
spindle-shaped creature, which
may reach a length of J- mm.
The front end is blunt and

Euglena.

Fig. 96.- -Euglena viridis ,
highly magnified.

av., Accessory contractile vac- , n , 1

tides ; cv., main contractile bears one flagellum rooted at
vaciioie ; chp., one of the ^ base of a funnel-shaped pit,

chloroplasts ; cu., pellicle; . r r

e.s., eye-spot ; ec., ecto- which IS kllOWU as tile gllllet

gdSt; md nud^i;;^|:; but probably never used as

paramylum granules ; p.g' SUCh. There is a Strong pellicle,
protoplasmic mass, with . . 1

paramylum granules, from a distinct eCtOSarC, and a

S^reservok15 l&StS radiate ; central, spherical nucleus.

Band-shaped, green chloro-
plasts (p. 24) radiate from a point in front of the nucleus,
where granules of the starch-like substance paramylum
accumulate. Waves of contraction pass along the body
(Fig. 97), but contractile strands (myonemes, p. 157)

FLAG ELLA TA

155

are. lacking. The vacuole system is complicated con-
sisting of a reservoir opening into the gullet, a contractile
vacuole which discharges at intervals into the reservoir,
and a number ot accessory vacuoles which surround
the main vacuole and re-form it. A red pigment spot
or stigma lies against the front side of the reservoir and
enables the working, of the flagellum to be regulated
b\ the amount ot light in the surroundings. Repro-
duction is by binary fission, beginning at the front end.
the nucleus undergoing a peculiar mitosis. It may take
place in free individuals alter the loss ot the flagellum,
or in a gelatinous cyst, within which it may be repeated
several times. The occurrence of syngamy is extremely
doubtful. 1 he nutrition ol Euglena is normally that of

FlG. 97. — Euglena viridis ,

A, A A", Three positions of the body.

a plant, though it is better il the water contain a little
nitrogenous organic matter. If the solution be rich in
such matter, certain species (but not E. viridis ) can, and
it light fails them do, nourish themselves entirely from it.
In darkness the chloroplasts of these species grow Dale
and shrink.

Copromonas (or Scytomonas , Fig. 4) is a flagellate which
Copromonas. llves in moisture of dung. It is related to
Euglena but colourless, as Polytoma is a
colourless Chlamydomonas . It nourishes itself, however,
not as Polytoma does by absorbing through its surface
the products ot decomposition amid which it lives, but
by swallowing through its gullet the bacteria which live
in the same solution. Its syngamy is performed solely by
fully-grown ordinary individuals. Usually the syngamy
takes place when the dung is becoming uninhabitable for

MANUAL OF ELEMENTARY ZOOLOGY

156

the Copromonas , and the zygote becomes encysted. From
this condition it only emerges in fresh dung, to reach
which it must be swallowed in contaminated food by a
frog, and passed intact with the faeces.

Per cinema, common in stagnant water, is another colour-
peranema. ^ess relah°n of Euglena . It is larger than
Copromonas , pear-shaped at rest but very
active in changing its shape, has one flagellum, rooted in
a reservoir which opens in front of the gullet, and feeds
by swallowing smaller organisms into the gullet, the wall of
which is strengthened by stiff rods. Probably Peranema
is also saprophytic (see below).

The organisms which we have been discussing in this
chapter exhibit all the three types of nutrition
Nutrition. practised by animals and plants. In Chlamy-
domonas simple inorganic substances are
absorbed through the surface, and from them complex
substances are manufactured by means of the energy of
the sun’s rays. Such organisms are said to be holophytic.
In Copromonas and Peranema complex organic substances
are taken in through a mouth, after the manner of animals.
Such organisms are said to be holozoic. In Polytoma ,
organic substances are absorbed in solution through the
surface of the body. Such organisms are said to be sapro-
phytic. The substances which form the food of various
saprophytic organisms differs a great deal. In Polytoma
they are relatively simple (amino-acids, acetates, etc.).
Euglena , which is both holophytic and saprophytic, prefers
for its saprophytism more complex compounds. Many
parasites in the alimentary canals of animals nourish
themselves saprophytically on the digested food of their
holozoic hosts.

In later chapters (pp. 183, 200), there will be found
Fiageiiata descriptions of the organisms known as
Trypanosoma and the Choanoflagellata , which
resemble those described in this chapter in the possession
of flagella and in certain other respects, and with them are
classed by zoologists as Fiageiiata or Mastigophora. The
green flagellates are, as we have seen, closely related to
plants, and it is natural that botanists claim the Fiageiiata
as members of the Plant Kingdom.

CHAPTER VIII

MONOCYSTIS

General

Features.

Among the organs of reproduction of an earthworm are
certain sacs, known as the vesiculae seminales,
in which the sperm ripens. Here are generally
to be found specimens of the parasites known
as Monocystis , which live by absorbing, through the surface
of their body, the fluid in the vesiculae which is provided
for the nourishment of the spermatozoa. Two kinds ot
these creatures may be present, differing in size and in
certain other particulars. The larger kind, M . magna, is
easily visible to the naked eye as white threads, hanging by
one end from the funnels of the vasa deferentia (see p. 252).
The smaller, known as M. lumbrici , is more often found free
in the fluid among the developing spermatozoa. The body
of a full-grown Monocystis is long and narrow, and consists
of a soft, granular endoplasm and a firm, clear ectoplasm.
The endoplasm contains numerous granules, many of which
consist of the carbohydrate substance paraglycogen , and the
ectoplasm is covered with a stout cuticle and has in its
deeper layer a network of contractile threads, the myonemes.
While the cuticle makes it impossible for the protoplasm to
flow out into pseudopodia, the myonemes enable it to
change its shape by squeezing the fluid endoplasm from one
part of the body to another. Slow waves of contraction of
this kind are constantly passing along the body. In the
endoplasm there is a large nucleus, but there is no con-
tractile vacuole. At one end of the body an indefinite
knob enables it to adhere to one of the cells of the
funnel.

In the stage which we have just described, the animals

I57

158 MANUAL OF ELEMENTARY ZOOLOGY

are known

^Reproduction,,

Around this
individual

mass a
now divides

Fig. 98. — Monocystis.

M. magna ; B, M. lumbrici.
The latter is covered with
the tails of spermatozoa, the
offspring of the sperm mother
cell in which it was embedded.

as t rop/iozoites. When they are full grown,
two of them come together and form them-
selves into a rounded mass without fusing.

double cyst is secreted. Each
by multiple fission, in which the
mitosis resembles that of the frog
in that the centrosome appears
outside the nucleus and the nuclear
membrane disappears. There
arise thus, as in the spore forma-
tion of Amceba, a number of small
germs, a certain amount of residual
protoplasm being left, which is
absorbed by the germs during
their development. 'The germs
unite in pairs, in which one
member is derived from each
parent. Thus, although the par
ents are to all appearance exactly
alike, there happens here what
happens also in the frog, where
the parents are unlike, namely,
that the gametes are derived from
distinct parents. This is known
as cross-fertilisation , and is found
in the vast majority of cases
throughout the animal kingdom,
though instances do occur of what
is known as self-fertilisation , in
which gametes derived from the
same parent unite.

It is said that in M. magna the
germs from the two parents are
alike, but in M. Iu7?ibrici those of
one parent — the “ female ” — are
rounded, and those of the other —
the “ male ’—pear-shaped. Each
zygote is known as a sporont-, it now
secretes a boat-shaped, horny case,
and is known as a fseudonavicella .
Within the case it divides by

MONOC Y ST IS

159

repeated fission into eight sickle-shaped sporozoites. There
are thus two generations of spores 1 in the life history

Fig QQ' — The life-history of Monocystis. — After Butschli.

1. Young individual (c) lying within a sperm mother cell of an earthworm.
2 Association of two individuals within a cyst, ready to form gametes.

3. Numerous spore-cases (sft.c., pseudonavicellae) within a cyst.

4. A spore-case with eight spores ( sp .) and a residual core (rt?.).

Fig. ioo. — Part of a cyst of Monocystis lumbrici showing the two kinds
of gametes and the residual protoplasm of one ot the parents. —
After Ploffmann.

of Monocystis. No further development takes place
until the pseudonavicellae get free from the worm, which
they generally do by the destruction of the latter.2
Probably this takes place by its being eaten by a bird,

1 A spore is a small reproductive body formed by multiple or re-
peated fission. It may or may not be a gamete. If it be enclosed
in a case it is known as a chlamydospore ( e.g . pseudonavicellae),
if it be naked, as a gymnospore {e.g. spores ot the Amoeba shown in
Fig- 92)- Amoeboid spores are known as amcebulcB or pseudopodio-
spores, flagellate spores as flagellulcB or flagelhspores .

2 The details of the transference of the spores of Monocystis are very
imperfectly known. It is said that occasionally they pass from one
worm to another during coition, but this is believed not to be the
usual method, if indeed it be effective at all.

i6o

MANUAL OF ELEMENTAL Y ZOOLOGY

Protected by their horny cases, the sporozoites pass
through the gut of the bird and are distributed in its
droppings over the soil, where they are washed down by the
rain and presently swallowed by another worm with the earth
from which it obtains its food. The spore-cyst is dissolved
m fbe intestine of the worm, and the sporozoites come out
and bore their way through the wall of the gut and other
tissues till they reach the vesicuhe seminales. Here each
enters a sperm-mother-cell, where it grows by absorbing the
protoplasm which is meant to serve for the nourishment of
the spermatozoa (see p. 273). The latter are formed, but
wither, their tails- only remaining attached to the young
Monocystis , which looks as though it had a coat of cilia.
Finally they disappear, while the Monocystis continues to
grow. Thus the sporozoites become trophozoites by
development. 3

In a later chapter (p. 188) there will be found
descriptions of animals related to Monocystis which are
parasitic in the body of man, where they are the cause of
malarial fevers.

CHAPTER IX

PARAMECIUM AND VORTICELLA. PROTOZOA

Paramecium caudatum , the Slipper Animalcule, is a
minute animal found in water in which dead
GeneraT?kfm leaves or other remains of organisms are decay-
Features, ing. The decay is brought about by bacteria,
and upon these the slipper animalcules feed. A
rich culture ot Paramecium may be obtained by steeping hay
in water, allowing it to decay, and adding to the mfusion thus
made mud or weeds from a freshwater pond which contains
Paramecium. The animals may easily be seen with the
naked eye as minute, greyish white, oblong creatures, shoot-
ing swiftly about in the water. The body of Paramecium
is spindle-shaped, somewhat flattened on one side, and
with one end blunter than the other. The flat side is called
“ ventral” and the blunt end is anterior. This end appears
as though it had been twisted, so that a groove which it
bears is spiral, starting in front on the left and curving
round to the ventral side, where it is continued back in
the middle line to within about a third of the length of
the body from its hinder end. The groove is known as
the peristome : from its hinder end there passes backwards
into the body a funnel-shaped gullet or vestibule , the open-
ing from vestibule to endoplasm being known as the mouth
The whole body is covered with fine protoplasmic threads
of the kind known as cilia (see p. 109) by whose lashing the
animal swims and gathers its food. The cilia are set at
equal distances in rows, which run lengthwise in the hinder
part of the body, but follow the spiral twist in front : they
also line the gullet, where two or three rows of them are
fused to form an undulating membrane which hangs from
the roof. The cilia work regularly in waves, lashing back-
II 1,6 1

162

MANUAL OF ELEMENTARY ZOOLOGY

wards (Fig. 95, D) and driving the blunt end of the animal
forwards, with a rotating movement like that of a rifle
bullet owing to its spiral shape. The animal can encyst.

Paramecium , like Amceba , Polytoma , and Monocystis ,
is not divided into cells. It has a soft, gran-
Ectopiasm ular endoplasm and an ectoplasm which is
Endoplasm. firm and gives the body its shape, but elastic, so
that the animal can bend and squeeze through
narrow gaps. The outermost layer of the ectoplasm is

u.m. g. an.

Fig. ioi. — Paramecium caudatum.

A, An individual seen from the left side, highly magnified; B, a diagrammatic
view of an individual from the ventral side, less highly magnified.

an., Position of temporary anus ; c.v., contractile vacuole ; ec., ectoplasm with
trichocysts ; f.v., food vacuoles ; g., gullet ; meg., meganucleus ; mi., micro-
nucleus ; pst., peristome ; u.m., undulating membrane.

a tough pellicle. Below the pellicle comes the cortex , a
thicker, clear layer of ectoplasm in which are embeddied
peculiar structures known as trichocysts. These are
spindle-shaped bodies with a fine point, and consist of
some semi-liquid substance. They are placed at right
angles to the surface, with the point in the outer part
of the layer. If the animal be stimulated by impact or
by a solution of some irritating substance, they suddenly
elongate and project from the body as threads, of which
the points are sticky while the rest has hardened. The
trichocysts are organs of adhesion by which the animal

PARAMECIUM

16-

anchors. When it breaks free the threads are lost and the
trichocysts replenished. The pellicle is marked by rows
of rectangular or hexagonal pits, in each of which a pair
ot cilia arise, while the trichocysts lie under the transverse
ridges between the pits. Each
cilium consists of an axial thread
and a covering layer continuous
with the pellicle. The axial
thread stops short of the tip
of the cilium, which is pointed.

Below the cilium the thread is
continued inwards into the cor-
tex, within which it bears a swell-
ing known as the basal granule.

The basal granules are united
by a system of threads known
as neuronemes which are possibly
conductile. The endoplasm con-
tains numerous granules, some of
which appear to consist of waste
matters ready tor excretion, while
others may be stored nutriment.

Glycogen is diffused through the
endoplasm.

Paramecium caudatum has two
nuclei. These, how-
ever, are not both of
the same kind, like the nuclei of a
Pelomyxa (p. 147), but consist
of portions ot the nucleoplasm
specialised for different purposes.

One is large and is concerned with
the ordinary life of the body. This
is known as the meganucleus. The
other is small and is specialised
for the purpose of conjugation. This is the micronucleus.1
We may roughly compare the meganucleus with the nuclei
of the body-cells of the frog and the micronucleus with
the nuclei of the germs. The nuclei lie in the endoplasm

1 The species known as Paramecium aurelia has two micro-
nuclei.

Nuclei.

Fig. 102. — A portion of
the surface of Para-
mecium, very highly
magnified.

Above, a portion of the
surface in vertical section.
Below, portions of two
cilia enlarged, to show
axial filaments.

ax, Axial filament ; b.g., basal
granule of a cilium ; ci.,
cilium ; cx., cortex ; en.,
endoplasm ; gr., granules
at angles of meshes ;
p., pit in the sculpture of
the pellicle ; pel., pellicle ;
tri., trichocysts.

164

MANUAL OF ELEMENTARY ZOOLOGY

above the gullet, the micronucleus in a cleft in the side of
the meganucleus.

1 here are two contractile vacuoles , which lie in the cortex
Excretion. °f t^ie dorsal side, one towards each end. At
its full size each is a large spherical space
surrounded by from six to ten pear-shaped radiating canals,
whose wide ends lie under it. These are the formative
vacuoles. Contraction or “ systole ” affects only the central
vacuole. Alter it has taken place, the formative vacuoles
flow together at their inner ends and thus form the

beginning of a
new contractile
vacuole, round
which new canals
appear, starting
as mere slits and
swelling to a
pear shape by the
enlargement of
their inner ends.
Over each con-
tractile vacuole
there is a minute
gap in the pellicle,
through which
the contents of
the vacuole are

Fig. 103. — Successive stages of the contractile discharged. It is
vacuole of Paramecium. Stated that the

supposed excre-
tory granules of the endoplasm collect near the forma-
tive vacuoles and are gradually dissolved. If so, it
may be that they are removed by the vacuoles. In any
case these have the same water-regulatory function as
those ot Amoeba. Urea is said to accumulate in cultures
ot Paramecium. Possibly it has been excreted by the
general surface of the body.

The food consists of bacteria and other minute organ-
Nutrition. isms. These are drawn towards the mouth
by the current set up by the cilia of the
peristome and driven down the gullet by the working

PARAMECIUM

165

l. C.

s. c.

of the undulating membrane, which has a waving motion.
The pressure of the water driven into the gullet with the
food particles causes the naked endoplasm at the bottom
of the gullet to bulge inwards, and into the space thus
formed the food is forced. A drop of water containing
the food particles is now pinched off by a contraction
of the endoplasm and becomes a food vacuole, which
is carried by a streaming of the endoplasm around the
body, passing first backward along the ventral side, then
forward nearly to the middle of the body, then through
several turns of a short circuit in this region of the body,
and finally forward to the front end and back so as to
complete the circuit of the body. During these wanderings
the food is digested. The undigested remains are then
expelled at a spot just
behind the end of the
gullet, where a passage
through the ectoplasm,
known as the temporary
anus, is formed when it

IS required, two periods Fig, 104. — A diagram of the course of
may be recognised in the the circulation of the food vacuoles
digestion. In the first in Paramecium.
period the water taken l.c., Long circuit ; s.c., short circuit,

in with the food is being

absorbed. Substances are secreted into the vacuole during
this period which give it an acid reaction and kill the
prey. In the second period an alkaline digestive juice
is secreted into the vacuole, which increases in size. It
appears that Paramecium cannot digest fat.

Like all other organisms, Paramecium has automatism
(p. 8). Its incessant activity is spontaneous
so far as immediate external stimuli are con-
cerned, but is continually modified by such
stimuli. The movements of Paramecium are much more
active and definite than those of Amoeba , and it is corre-
spondingly easier to observe the effect of various stimuli
upon the animal. These effects are of two kinds, upon the
rate of movement and upon its direction. (1) Many acids,
alkalies, salts, and other substances in dilute solutions
cause an increase in the rate of motion owing to a more

Effect of
Stimuli.

1 66 MANUAL OF ELEMENTARY ZOOLOGY

rapid working of the cilia. Moderate increase of tempera-
ture has the same effect. On the other hand, dilute
solutions of narcotics, such as alcohol, ether, or chloroform,
cause the cilia to work more slowly. All these reactions
are probably merely the direct effects which such stimuli
are known to have upon protoplasm. (2) In order that the
effect of stimuli upon the direction of movement may be
observed, it is of course necessary that the stimulus should
fall unequally upon different sides of the animal. It will
then move to or from the direction in which the stimulus
is strongest. This can be arranged by placing with a fine
pipette a small drop of some solution in the vessel in
which the animals are confined, or by heating or lighting
one side only of the vessel. Paramecium will move
towards weak acids or moderate warmth and away from
alkalies, strong acids, warmth above 250 C., etc. Such
movement is known as taxis. It was believed that it
could be explained in a simple way by the supposition
that the effect of the stimulus in each case was either
to slow or to quicken the working of the cilia on the side
nearest to it, so that the animal was driven mechanically
either towards or away from the stimulus by the unequal
working of its cilia. What really happens, however, is by
no means so simple. The effect of all stimuli to which
Paramecium reacts naturally is to repel it. The animal on
receiving a stimulus first withdraws, by a definite backward
movement due to a reversal of the working of its cilia, from
the stimulus. It then turns towards the dorsal side and
swings the front end of its body round in a circle with that
side outwards so that it comes to point in a new direction,
and in that direction it swims forwards unless it again
meets the stimulus. Thus its approach to conditions which
appear to attract it is in reality due to an avoidance of the
relatively less agreeable conditions which it meets in other
directions during automatic wanderings. It behaves as if
it were “ trying ” different directions of movement till one
is found from which it is not repelled. It is claimed that
this procedure, known as the method of trial and error ,
can be discerned in the behaviour of all animals, from
Amoeba (see p. 144) upwards.

Paramecium reproduces by binary transverse fission.

PARAMECIUM

167

The meganucleus divides amitotically, the micronucleus

Reproduction. b>r a mitosis in which, as in that of Amoeba , the
nuclear membrane does not break up, and
the place of centrosomes is taken by pole plates. Mean-
while a groove appears round the middle of the body and
deepens till the cytoplasm is sundered into two, each half
containing a daughter nucleus of each kind and one of

10°- 25°-

Fig. 105. — The reaction of Paramecium to heat
and cold. — From Jennings, after Mendelssohn.

At a., the Paramecia are placed in a trough both ends of which
have a temperature of 190 C. They are equally scattered.
At b., the temperature of one end of the trough is raised
to 38° C., while the other is only 26° C. The Paramecia
collect at the end which has the lower temperature. At
c., one end has a temperature of 250 C., while the other is
lowered to io° C. The animalcules now collect at the end
which has the higher temperature.

FlG. 106. — Paramecia collecting in a drop of -Tj- per cent
acetic acid. — From Jennings.

MANUAL OF ELEMENTARY ZOOLOGY

1 68

the. contractile vacuoles. The two bodies formed by this
fission are like those of Amoeba , asexually produced
young, analogous to the buds of certain higher animals of
which we shall speak in a later chapter (p. 221). Their
development involves not only growth but also the re-
modelling of the body, since each of them lacks half the
outward organs of the parent, while those which it has are
too large for it. In a well-fed culture, division takes place
two or three times a day, but ll the animals be ill-nourished
it is much less frequent, and if they be starved they cease
to divide.

The conjugation of Parai?iecium is a remarkable process,
conjugation,, °f a kind found only in this creature and in
those which nearly resemble it. In it gametes
are formed and become rid of the body nuclei of their
parents, without being set free as they are from the
cellular body of the frog. The individuals which form the
gametes are exactly alike and resemble normal individuals,
except that they are somewhat smaller. As a rule, the
process begins during the late hours of the night’ and
lasts till the next afternoon. The details are as follows :
Two individuals, which we will call conjugants / come to-
gether as. those of Monocystis do, but without encysting,
and lie with their ventral sides touching, the endoplasms
becoming continuous in the region of the gullets, which de-
generate. We may compare this with coition. The micro-
nucleus of each conjugant leaves its normal position, lies
free m the cytoplasm, and grows larger. It then divides
twice, and three of its four products degenerate. During
these divisions the number ol chromosomes is halved, as it
is in the gametogenesis of the frog (p. 131), though the
details of the process differ in the two cases. The remain-
ing micronucleus divides again, this time somewhat
unequally, the smaller product being the male pronucleus,
the larger the female pronucleus. At this stage we may
regard each conjugant as containing two gametes, repre-
sented by the two pronuclei. These are analogous to an
ovum and a spermatozoon, so that the animal maybe said
to be hermaphrodite. The true syngamy now takes

1 They are often alluded to as gametes. This is incorrect. They
are not gametes, but parents which form gametes.

PARAMECIUM

169

place. The male pronucleus of each conjugant passes
over into the other and fuses with the female pronucleus
of the latter. The body which belonged to each
conjugant comes thus to contain a micronucleus of
mixed origin. It is, in fact, a zygote. The zygotes
separate and are known as exconjugants . During conjuga-
tion the meganucleus degenerates, splitting up into shreds,

which disappear. Thus
the meganucleus resem-
bles in the fact of its
mortality the body-cells
of the frog, though the
body as a whole has the
immortality of a germ-
cell or an Amoeba. After
separation the joint
micronucleus of the ex-
conjugant undergoes a
development whereby
nuclei of both kinds aie
provided. It divides
three times successively,
so that the body con-
tains eight nuclei. After
an interval the body
divides into two, each

Fig. 107.-A diagram of the behaviour half, . containing four
ot the micronuelei during the con- tlUClei, and after a fur-
jugation of Paramecium caudatum. ther interval these halves
The white circles represent the divide, so that there are
portions wh.ch degenerate. four individuals, each

See also Fig. 108, A. with two nuc|ej,

of which becomes a meganucleus and the other
nucleus.

1 he conditions under which conjugation takes place in
Paramecium have been, and are still, the subject of much
investigation. Many points still remain to be cleared up,
but certain results have now been reached. Conjugation
generally occurs at the beginning of a falling off in the
supply of food after a period of exceptional plenty that has
brought about rapid multiplication. Thus it will often take

one
a micro-

B

Fig. 108.— Conjugation in Ciliata.

A, Paramecium ; B, Vorticella.

c., Pseudo-female conjugant ; c ., pseudo-male ; me., me meganuclei • mes: disin-
tegrating fragments of meganucleus ; mi., micronuclei mi ., abortive micronuclei.

170

PARAMECIUM

171

place in an infusion in which the bacteria, having used up
the nourishment provided by the plant-remains, are falling
off in numbers, and thus the raramecia, after a plentiful1
supply of food, are beginning to experience dearth. But
there are some races in which it is difficult to bring
about conjugation, others in which it has never been seen,,
and yet others in which it takes place at short intervals-
without apparent cause.1

In a stock or “culture” of Paramecium kept in the
laboratory, it often happens that after a time
Depression. ajj members pass into a state of “depres-
sion,” in which they have an overgrown meganucleus and a

Fig. 109.-—S emidiagrammatic views of individuals of Paramecium

caudalum.

A, In depression ; B, in conjugation ; C, in fission.
g., G.iUet; meg., meganucleus; mi., micronucleus.

stunted body, divide more slowly, and show an increasing
degeneration in various organs and functions of the body.
At last they are unable to digest food and die. Depression
is an abnormal event, produced by unnatural conditions
of culture. In its earlier stages the animals can be-
stimulated in such a way as to endow them with a new

1 It has been said that descendants of the same exconjugant will not
conjugate, and that individuals from another stock must be introduced,,
but this has been disproved.

172

MANUAL OF ELEMENTARY ZOOLOGY

lease of life. This is said to happen if conjugation occur.
It can be effected by shaking the culture, or better by a
change of diet, as by feeding with beef-tea. After a time
the stock can be put back to a diet of hay bacteria and
kept till there sets in a deeper depression, which is capable
of being averted in the same way. By one means and
another (after a while beef-tea failed, and brain and
pancreas extracts had to be used) the life of such a culture
has been kept up for two years, but the effect of unnatural
conditions was in the long run too strong, the recurring
periods of depression became more and more severe, and
at last the whole brood died. Depression has been re-
garded as the old age of the stock, and the alleged
averting of it by conjugation has been compared with
the renewal of the lease of life in the young of multi-
cellular animals produced from a fertilised ovum. Actu-
ally depression is a disorder brought on by unnatural
conditions of culture, but it is true that even in the best
conditions the vitality of a stock of a ciliate protozoon,
as estimated from its rate of division, periodically wanes
and is restored, and that its restoration coincides either

with conjugation or with the occurrence in solitary
individuals of an internal process known as endomixis
which resembles conjugation but does not involve syn-
gamy. Both these processes renew the meganucleus, and
it is probable that their effect is due to this, just as, in
multicellular animals, the renewed vitality of the young
is due to the nuclei and cytoplasm in their bodies being
newly formed.

Among the most beautiful forms of pond life are the
bell-animalcules, of which the scientific name
is V or ticella. Various species of these creatures
may be found as minute, colourless bodies
fastened to weeds by stalks which contract at
the slightest disturbance of the water. Some of them also
appear in infusions, d he body of a J or ticella is outwardly
shaped like a bell, but has no hollow within, the bell being
filled with a mass of protoplasm. In the place of the
handle is a long stalk, by which the animal is fastened to
some solid object. Animals which are thus fixed are said
to be sessile . The bell can be bent upon the stalk. The

Vorticella

General

Features.

VORTICELLA

173

wide end of the bell has a thickened rim , within which is a
groove, the peristome. On one side there passes from
the peristome, down into the mass that fills the bell, a
tube which is the gullet. The first part of this is wider
than the rest, and the name vestibule is sometimes re-
stricted to it. The part of the upper surface which
is encircled by the peristome is known as the disc. It

Ftg. i 10.— A group ot individuals of Vorticclla
in various phases of the life-history.

<7., Ordinary individual ; b., the same contracted ; c ,,
ordinary fission ; d., a later stage of the same ; e.,
free-swimming individual produced by ordinary
fission _/" ., two modes of fission to form a con-
":ugant ; g., conjugation.

is not level, but slopes, being raised on the side where
the gullet lies. The disc can be retracted, and the rim of
the peristome drawn inward over it. Around the edge of
the disc and down into the vestibule two rows of cilia
wind spirally counter clockwise, the inner long and upright,
the outer short and slanting outwards. In the vesti-
bule the members of the outer row are fused to form an
undulating membrane. There are no cilia elsewhere upon
the body.

a 74

MANUAL OF ELEMENTARY ZOOLOGY

The general character of the ectoplasm and endoplasm is
the same in Vorticella as in Paramecium , but
Ectoplasm the pellicle of the bell-animalcule is sculptured
Endoplasm. in various ways according to the species, and
below it is a distinct alveolar layer. Just under
the alveolar layer, in the walls of its bubbles, is a layer of
very fine contractile fibres or myonemes. Near the stalk the

Fig. iii. — Vorticella , highly magnified.

an., Position of temporary anus ; c.f., contractile
filament ; c.v., contractile vacuole ; cut.st.,
cuticle of the stalk ; dsc., disc ; ec., ecto-
plasm ; f.v., food vacuoles; g., narrower
part of gullet ; i.ei., inner row of cilia ; meg.,
meganucleus ; mi., micronucleus ; myn.,
myonemes ; o.ci., outer row of cilia ; pst.,
peristome ; res., reservoir of contractile
vacuole ; rim ; u.m., undulating mem-

brane ; v., vestibule.

ectoplasm is much thickened and the myonemes pass
inwards through it to join in the middle, where they form
a central contractile fibre which, with a covering of ecto-
plasm, makes up the stalk. This is enclosed in a cuticular
tube formed by secretion. The contractile fibre is not
quite straight, but lies in a very open spiral, so that when
it contracts it draws the stalk into a close coil. There are
no trichocysts. The endoplasm is granular.

VOR TIC ELLA

175

Internal

Organs.

A meganucleus and a micronucleus are present, the former
a long, curved band, the latter small and placed
beside the meganucleus, usually in the upper
part of the body. There is a contractile vacuole,
which has no canals. It lies in the upper region of the body
and communicates with the vestibule through a reservoir ,
which has a narrow permanent opening. The contractile
vacuole contracts sharply at intervals, discharging into the
reservoir. The latter then contracts slowly, driving its
contents into the vestibule, but not itself disappearing.
Feeding and digestion take place much as in Paramecium.
The little organisms which serve as lood are collected and
driven into the gullet by the action of the cilia. The lood
vacuoles follow a definite, winding course in the body,
passing through stages similar to those in Paramecium.
The faeces are discharged into the vestibule by an anus,
which in some species is a permanent opening through
the ectoplasm.

The reproduction of Vorticella takes place by binary
fission, which is of two kinds — ordinary fission,
Reproduction. an(j qiat forms conjugants. In ordinary

fission, the rim closes in over the disc, the body becomes
shorter and wider, and the meganucleus contracts and lies
across the body, which then divides into two, the plane of
fission being in line with the stalk. The nuclei behave as
in Paramecium. One of the daughters remains upon the
stalk ; the other grows a circlet of cilia in the hinder
region, at the level at which the ectoplasm thickens, breaks
off, and swims away by means of its cilia, to settle down
elsewhere by the end which was attached to the stalk of
the parent. It grows a new stalk for itself. In this form
of reproduction the offspring are equal in bulk. In the
fission which forms conjugants the parent gives rise to one
large individual and one or more of a smaller size. The
small individuals may arise by unequal binary fission,
sometimes called budding, or by equal fission, followed by
division of one product into four by repeated fission.1 In

1 The various kinds of fission of Amoeba, J orticella, and animals
related to them (Protozoa, p. 177) may be classed as : (1) equal binary
fission (p. 146), (2) budding, (3) repeated fission (p. 152), (4) multiple
iisdon (p. 147).

176

MANUAL OF ELEMENTARY ZOOLOGY

Conjugation.

either ease the small individuals resemble the free product
of ordinary fission in all but size.

The small individuals thus formed swim away, and each
attaches itself by its hinder end to the lower
part of the body of one of the stalked indi-
viduals. Most of the organs of the small individual now
disappear, and the ectoplasm between the two conjugants is
absorbed into their endoplasm, which becomes continuous.
The meganucleus in each begins to break up and disappear.
Meanwhile the micronucleus of the small conjugant has
divided into two. Now the micronuclei of both conjugants
divide twice, so that the larger contains four and the
smaller eight micronuclei. In each case all but one ol these
perish and the survivor divides into two, which corre-
spond to the male and female pronuclei of Paramecium.
This division takes place while the two micronuclei are
lying in the region where the endoplasm of the conjugants
became continuous. One half of each micronucleus passes
into the larger conjugant, where the two fuse as male and
female pronuclei. The other half of each passes into the
smaller conjugant, but these halves, instead of fusing,
degenerate and disappear. The endoplasm of the small
exconjugant is now drawn into the larger, the ectoplasm
shrivelling up and falling off. It will be seen that the
conjugation of Vorticella takes place in the same way as
that of Paramecium , but that one of the two exconjugants
perishes and is partly absorbed by the other.1

Carchesium is a small freshwater animal whose body
consists of a number of members, each of which
arc ©sum. ^as the structure of a whole Vorticella. It
arises from a Vorticella-Yike body, by divisions like those
which take place in the ordinary reproduction of Vorticella ,
save that the division passes some way down the stem and
then stops, leaving the bells joined by their stalks. Thus
the body is increased by the addition of new members
which repeat the structure of the old. In that it increases

1 The student should beware of comparing the smaller conjugant of
V orticella with a spermatozoon and the larger with an ovum. Ova
and spermatozoa are gametes of unlike kinds. The conjugants of
Vorticella are unlike, hermaphrodite parents, each of which forms two
unlike gametes.

PROTOZOA

177

the number of energids in the body, this process resembles
cell formation, but the two cases differ in that the new
energids ot Carchesium all repeat the whole structure of the
first and inherit all its powers, whereas a cell is a portion
ot the body with peculiar characters and restricted powers,
d he whole body of a Carchesium is said to be a colony , and
its members are zooids. Reproduction is brought about by
the complete fission from the body of certain zooids, which
thus become asexually produced young (buds). Each of
these swims off, settles down, and forms by growth and
nuclear division a new colonial individual. ' Conjugation
like that of Vorticella also takes place.

The detailed study

Protozoa, which we
have made ot
Paramecium and Vorti-
cella has shown to what
an extent organisation
can be carried without
the division of the body
into cells. Ranging in
grade of organisation be-
tween the simplicity of
Amoeba and the com-
plexity of Carchesium
there is an immense
number of animals
whose structure is non-
cellular,1 These animals are known as Protozoa.
Cellular animals are known as Metazoa. Those Pro-

tozoa which move by means of pseudopodia are called
Sarcodma or Rhizopoda , those which move by flagella are
Mastigophora or Flagellata , those which move by cilia
are Cihata. Protozoa which, like Monocystis , have no
external organs ot locomotion, are parasites, and form
numerous spores, are known as Sporozoa. In comparing
Amoeba with the trog we noticed that the absence in the
former ot cells— that is, of energids in the body which
are specialised and therefore liable to natural death— led

1 They are often said to be unicellular, but this, as we have decided
(p. 149), is a misleading use of the term “ cell.”

12

178 MANUAL OF ELEMENTARY ZOOLOGY

to its being, in a certain sense, immortal. The same is
true of all Protozoa, although, as we have seen, most
Ciliata, in which there is a partial separation of body-
substance in the form of a special nucleus, do at times
purchase their immortality at the price of the loss of the
meganucleus, forming a new one from the micronucleus.

In the next chapter there will be found (p. 195) descrip-
tions of several Ciliata which are parasitic in the frog and
in man.

CHAPTER X

THE PROTOZOA AS PARASITES OF MAN

The interest which the study of the Protozoa has for man-
kind is not merely theoretical, in virtue of the remarkable
peculiarities of their organisation, but is very near and
practical, by reason of the fact that a number of them live
in the bodies of men, and that there they sometimes cause
very serious diseases. In this chapter we shall study briefly
examples, drawn from all the four classes of the group,
of which man is a host — that is, which he harbours as
parasites. In so doing, our attention must be given both
to facts wrhich, directly or indirectly, are of medical im-
portance, and to others which have wider biological
significance.

The several kinds of Entamoeba differ from Amoeba only

Entamoeba. *n t-^at ^ave no contractile vacuole.1

They have one or two large blunt pseudopodia,
chiefly composed of ectoplasm, and they are all parasites,
usually in the alimentary canal of one of the backboned or,
as they are called, “ vertebrate ” animals. E. coli. lives in
the upper part of the large intestine of man, feeding upon
the bacteria which infest that region, and also upon the
remains of the food of its host, which are probably of
little value. It is harmless, and possibly sometimes even
beneficial by keeping down the bacteria. Its life-history
differs considerably from that of Amoeba proteus. In the
intestine it reproduces by binary fission, and, as some of
the individuals are being passed down the gut and cast out
with the faeces, certain of them undergo another process.

1 A contractile vacuole has been found in one organism which has
been classed with the Entamoeba.

79

i So MANUAL OF ELEMENTARY ZOOLOGY

In this the nucleus — after proceedings in which some of
its chromatin is lost, while a large vacuole temporarily
appears in the cytoplasm — forms eventually two nuclei
while a cyst is being secreted around the body. The
two nuclei in the cyst divide into eight. The ordinary En-
tamoeba die in the faeces. So, it is said, do the cysts if the
faeces dry, but if they remain moist until'they reach water
or human food and are swallowed by a man the cysts
germinate in the intestine of the new host, the protoplasm
dividing and emerging as little individuals each with a
nucleus. By these the cycle is re-started.

Fig. WT).— Entamoeba. — After Fantham.

A, E. coli ; B, E. histolytica .

b.c., Ingested red blood corpuscle food vacuole ; nu., nucleus;

ps . , 1 pseudopodium.

Entamoeba histolytica , sometimes known as E. dy sentence,
also inhabits the human intestine. It varies much in size
but reaches greater dimensions than E. coli , from which
it also differs in being more active, in having a distinct
ectoplasm over the whole surface of the body, in taking up
strongly, while still alive, the stain known as “ neutral red,”
and in that the principal chromatic body or “ karyosome ”
of the nucleus is centrally placed. Unlike E. coli it
attacks the mucous membrane of the intestine, probably
by the secretion of an enzyme. It then penetrates the
blood vessels in the same way, and is carried by the

THE PROTOZOA AS PARASITES OF MAN 1S1

circulation to the liver, where it may set up abscesses. Its
act ion on the intestinal wall causes dysentery. It feeds on
tissues and also on red blood corpuscles, which E. coli
does not. E. histolytica is widespread in tropical and sub-

Fig. i 14. —The life-cycle of Entamceba histolytica .

I— III, Binary fission; a-f, encystment and multiple fission,
s, Round ng off; b, cyst in which the nucleus has divided; c, cyst in which the
second division has taken place ; d, emergence from the cyst ; e, free amoeboid
individual with four nuclei which lie close together* f, eight amoebulas formed
from e by a complicated process of division.

tropical countries, and is the cause of much sickness and
loss of life. An Entamceba of small size ( E . minuia), and
another known as E. tetragena , with a large karyosome in
the nucleus, are now known to be forms of E. histolytica
which arise in certain circumstances. Its life-cycle appears

i S 2 MANUAL OF ELEMENTARY ZOOLOGY

to differ from that of E. coli chiefly in the number of the
cyst nuclei, ot which there are only four, though after

Fig. i 15. — Entamoeba histolytica.

u and b, Amoebae as seen in fresh stools, showing blunt ectoplasmic
pseudopodia, non-contractile vacuoles, ingested red corpuscles,
and, in a, nucleus ; c, an amoeba as seen in a fixed preparation ;
d, section of wall of liver abscess, showing an amoeba of
spherical form. The rounded amoebae on this plate must not
be confused with the encysted form.

emergence these divide with the cytoplasm to form eight
little amoebae.

THE PROTOZOA AS PARASITES OP MAN 183

A flagellate protozoon known as Trypanosoma is re-
sponsible for various verv dangerous diseases

Trypanosoma, rr . 0

ot man and animals m warm countries.
Trypanosoma is parasitic in the blood and other fluids
of backboned animals, but also capable of living in an
invertebrate which sucks the blood of its vertebrate host,
and by this it is transferred from one vertebrate to
another. It has a worm-like body, about one-thousandth
of an inch in length, tapering towards the ends, but
more pointed in front than behind. The shape of the body
is maintained by a strong pellicle. A single flagellum
stands at the front end, and from its base an undulating

Fig. 1 16. — Entamoeba histolytica in the encysted
condition. — After Fantham.

membrane runs along one side nearly to the hind end.
The flagellum is continued as a strongly-staining thread
along the free edge of the membrane, and terminates
behind in a minute “ basal granule " or blepharoplast ,
embedded in the cytoplasm. By the working of the
undulating membrane and flagellum the animal swims
rapidly with a graceful wavy movement, either forwards
or backwards. There is no contractile vacuole. Near
the middle of the body is an egg-shaped nucleus, but
a smaller mass, which stains like the nucleus, stands
close to the blepharoplast. This smaller body has been
called the “ kinetonucleus,” and supposed to have some
function in connection with movement, while the nucleus
proper, known as the “ tropnonucleus v regulates the
other functions of the body. It is now known that the

1 84 MANUAL OF ELEMENTARY ZOOLOGY

so-called kinetonucleus is not ot the nature of a nucleus,
and has no influence upon the movements of the animal,
which take place equally well in races in which it has been
artificially caused to disappear. It is best called the
parabasal body . Its function is at present unknown.
Trypanosoma has no mouth, but nourishes itself, like

Fig. i 17. — Trypanosoma gambiense. — After various authors.

A, B, C, Slender, intermediate, and stumpy forms from man; D, latent body •
E, slender form from gut of fly; F, crithidial form from salivary gland of flv!
G, ripe form from proboscis of fly.

bpt., blepharoplast ; ft., flagellum ; k.nu., parabasal body (kinetonucleus) ; tr.nu.
trophonucleus ; u.m., undulating membranes.

Monocystis , by absorbing through the surface of its body
substances obtained from the juices of its host.

In spite of the immense amount of investigation of
which its medical importance has caused it to be the
subject, the life-history of Trypanosoma is not yet thoroughly
understood. In the case of T. gambiense , the cause of the
terrible “ sleeping sickness ” of West and Central Africa,
the following facts have been established. In the body of
an infected man the parasites live at first in the blood, but

THE PROTOZOA AS PARASITES OF MAN 185

presently make their way into the lymphatic glands, and
thence into the fluid of the spinal canal and cavities of the
brain. While they are in the blood alone the man suffers
from “ Gambia fever,” but when they reach the central
nervous system the drowsiness which is characteristic of

Fig. 1 1 8. — Trypanosoma gambiensc.

A stained preparation of the blood of an infected guinea-pig, showing blood

corpuscles and parasites.

sleeping-sickness comes on, and increases, with, presently,
a wasting of the body, till death almost inevitably results.
The individuals found in the human host are not all alike,
some being long and slender, some short and stumpy, and
some intermediate in shape. The thin forms are the
youngest, the animals growing stouter as they mature, and
becoming stumpy m succeeding generations. There are

1 86 MANUAL OF ELEMENTARY ZOOLOGY

also differences in size, due to age and to the fact that the
binary longitudinal fission by which reproduction takes
place is sometimes unequal. In fission first the blepharo-
plast, then the parabasal body, and finally the nucleus
divide, while the flagellum and membrane are doubled,
but probably not by division.

During the progress of the infection some of the trypano-
somes pass into certain of the internal organs of their host,,
especially into the spleen and lungs. There they lose their
flagella and become of an oval shape. In this condition
they show resemblances to the predominant phase of the
organism known as Leishmania , which is the cause of the
kala-azar disease and of Delhi boil. True Leishmania
stages, which presently revert to the flagellate condition, do'
occur in the life-cycles of other trypanosomes. It has been
supposed that these phases of T. gambieuse are of a similar
nature and that they revert and thus make good the loss of
flagellates in the blood when, as happens between the fits
of the fever, the flagellates are reduced in number by the
secretion of “ antibodies ” (p. 124) by the host. On this
theory they have been called “ lacent bodies.” It is more
probable, however, that they are individuals in a state of
degeneration.

The invertebrate responsible for the spreading of Trypano-
soma gambiense is a fly, Glossina palpalis, related to the
tsetse fly, G. morsitans , which spreads another trypano-
some, T. brucei , the cause of the South African cattle
disease. Glossina sucks the blood of various backboned
animals — cattle, antelopes, birds, and so forth, as well as
man — and thus takes into its stomach such parasites as may
infest the blood vessels of its victims. When the object of
the attack of G. palpalis is infected with the trypanosome
of sleeping-sickness, the insect becomes capable of inocu-
lating a new host in the course of its feeding. The power
of inoculation is soon lost, but is regained after about
twenty days. It seems probable that the first inoculations
are made with trypanosomes which are still fresh in the
proboscis of the insect, but the later ones with individuals
which arise from the stumpy forms after passing through
a course of development in the insect’s alimentary canal
and salivary glands. During this development the stumpy

THE PROTOZOA AS PARASITES OF MAN 187-

forms become first long and slender, then, attached to the
wall of the salivary gland, pass through a “ crithidial phase ,r
in which the membrane starts in front of the nucleus, and
finally as stout-bodied, mature individuals are injected with
the saliva when a new victim is bitten.

Besides those that we have mentioned, there are known
a number of other trypanosomes — T. rhodesiense which
causes a sleeping-sickness in South Central Africa,
T. equinum which causes a horse disease in South America,
T. cruzi , the cause of a disease in children in the same con-

Fig. 1 19. — The tsetse flv Glossina palpalis. — From Thomson.

tinent, and so forth. Many, perhaps all, of these have a
wild host in which they are harmless, though in the
unaccustomed bodies of men or domestic animals they are
highly dangerous. Formerly no treatment was of any avail,
against them ; recent research has produced several synthetic
drugs (Germanin, Tryparsamide, etc.) which can cope with
at least the African species, but the best wav to combat them
is to avoid the attacks of the insects which transmit them.
Thus the clearing around places frequented by human
beings of the bush which is the haunt of Glossina has led
to a decrease in the number of cases of sleeping-sickness.

A much more widespread though less dangerous type of

MANUAL OF ELEMENTARY ZOOLOGY

1 88

disease than sleeping-sickness is malarial fever. This is
brought about by a minute protozoan parasite
Parasites. known as Hccmamocba or Plasmodium, 1 belong-
ing, like Monocystis , to the Sporozoa. The
dangerous stage of the parasite corresponds to the tropho-
zoite of Monocystis. It lives in the red blood corpuscles,
and is at first a round body with the appearance of a ring,
owing to the presence of a large (non-contractile) vacuole
in its middle. It has a single nucleus and no mouth, and
must absorb food from its surroundings through the surface
of its body. As it grows, it loses the ring-like appearance,
puts out pseudopodia, and forms in its cytoplasm granules
of pigment which is no doubt derived from the haemoglobin
of its host. When it is ready to reproduce, it is known as
a meront or schizoid. Its reproduction, called merogony or
schizogony , takes place by multiple fission. The pseudo-
podia are withdrawn and the nucleus divides repeatedly
till there are present some sixteen smaller nuclei. These lie
in the outer part of the body, and most of the cytoplasm
now gathers round them so as to form a rosette of little,
uninucleate individuals-— the merozoites or schizozoites — *
which surround some “ residual protoplasm ” containing
the pigment granules. Next the shell of the red corpuscle
breaks up, setting free the merozoites into the plasma,
where each of them proceeds to infect a new corpuscle,
into which it bores its way with a pointed end.

The time which is required to repeat this cycle of asexual
reproduction varies with the species of parasite. Thus of
the three kinds (at least) of Plasmodium which infest man,
P. vivax sets free a generation of merozoites in forty-
eight hours, P. malarice in seventy-two hours, and P.
falciparum at irregular intervals. The attacks of fever
occur when the corpuscles break up, probably because
there are then set free substances formed during the meta-
bolism of the parasite which prove poisonous to the host.
So it comes about that the fever caused by P. vivax returns
every third day, and is known as “ tertian ague,” and that

1 It is unfortunate that this name is also in use to denote a type of
relation of nuclei to cytoplasm — namely, that in which a syncytium is
formed by the fusion of free energids (p. 147) — which, as it happens,
is not found in the malaria parasite

Fig. 120. — The life-cycle of Plasmodium vivax.

-7 Merogony, asexual reproduction which takes place in man ; 8 13, gamoeony,

’ or sexual reproduction , which takes place in the stomach of a mosquito ; 14 20
SDorogony asexual reproduction in the body cavity of the mosquito.

SP reinfection of a red corpuscle ; 2, signet-ring stage ; 3 , amoeboid stage ; 4> fnll-
erown meront preparing to divide ; 5, multinucleate stage ; 6, rosette stage,

^Assuissss-:

BiK sk by latter

189

a 90 MANUAL OF ELEMENTARY ZOOLOGY

-caused by P. malaria (quartan ague) returns every fourth
day, while P. falciparum causes irregular (quotidian) fevers
which are more or less continuous. These latter are the
pernicious malaria ’ of the tropics. For about ten days
alter infection the parasites are not numerous enough to
■cause serious trouble. This period is known as the “ period
ot incubation.” Many generations of merozoites may
succeed one another during the course of the illness, but
-eventually the resisting powers of the host begin to get the
-better of the infesting organisms, or, on the other hand,

the patient may be about to die.
In either case it behoves the parasite
to arrange for the continuance of
its race elsewhere. This is done bv
the provision of a fresh kind of in-
dividual, adapted to transmission
by mosquitoes to new human hosts.
These, because they give rise in the
mosquito to gametes, are known as
gamonts , or gametocytes, though
the latter name more properly be-
longs to cells of similar function in
the bodies of Metazoa (p. 131).

The gamonts begin to arise when
the period of incubation is past.
In the parasites of tertian and
quartan fevers they are rounded,
in that of pernicious malaria
crescent-shaped. They are larger
than the schizonts and have more
of the dark pigment.

It is said that they have no ring-stage in their develop-
ment. They remain in the corpuscle where they arose from
a merozoite, and undergo no change unless they be sucked
in by a mosquito ; but in that case, whereas all other forms
of the parasite die and are digested bv the mosquito,
the gamonts, becoming tree by the breaking up of their
■corpuscles, proceed to develop gametes. They are of
two kinds, male and female, the former with a larger
nucleus and more lightly staining cytoplasm than the
fatter. In the male gamont the nucleus breaks rapidly into

Fig 121. — Gamonts of
Plasmodium falciparum .

■a, Before taking on the sausage
shape ; b1, male gamont in
sausage stage; b"-, female
gamont in the same stage.
The outline is that of the
red corpuscle.

%

Fig. i 22.' — Plasmodium vivax , the tertian ague parasite. —
From Muir and Ritchie.

e4, Several young ring-shaped amoebulae within the red corpuscles, one of the latter
enlarged and showing a dotted appearance ; B, a larger amoebula, containing
pigment granules ; C, two large amoebulae, exemplifying the great variation in
form; D, large amcebula assuming the spherical form and showing isolated
fragments of chromatin, preparatory to schizogony ; E, schizont which has pro-
duced eighteen schizozoites (merozoites), each of which con’ains a small collec-
tion of chromatin ; F, merozoites set free. ( X ioco.)

191

192

MANUAL OF ELEMENTARY ZOOLOGY

some half-dozen fragments, leaving a residual mass in the
central cytoplasm. The daughter nuclei come to the
surface, and grow out, with a suddenness which is almost
explosive, into fine threads of nucleoplasm, projecting
from the body in scarcely perceptible sheaths of cytoplasm.
These are the microgametes. They lash violently, dragging
about residue of the gamont body, till they break free.
The remains of the gamont perish. The female gamont, by
a process in which the nucleus loses a part of its contents,
becomes a single macrogamete. It is now ripe for fertilisa-

tion by a microgamete
which penetrates the body,
and the nuclei (male and
female pronuclei) fuse.
The zygote changes from
a rounded to a worm-like
creature, which glides
about by contractions of
its body, pierces the epi-
thelium of the insect’s
stomach with one end,
which is pointed for the
purpose, and comes to rest
in the sub-epithelial tissue,
where it rounds itself off

\

Fig. 12 3.— A mosquito {Anopheles).

— From Lankester’s Zoology. and forms a cyst. It is

known as the sporont on

account of its further history, which is as follows.

Through its thin cyst-wall the parasite continues to
absorb nutriment, and grows in size, bulging out the wall
of the stomach into the body cavity so as to form a kind
of blister. As it grows, its nucleus multiplies by binary
fission and cytoplasm becomes concentrated round each
nucleus to form a body known as a spor oblast. Now the
nucleus of each sporoblast divides repeatedly and the
surface of the body grows out into slender processes, into
each of which one of the daughter nuclei passes. Finally
the processes break off, and so the cyst contains hundreds
of needle-like sporozoites together with some residual
protoplasm. The ripe cysts burst and scatter their con-
tents into the body cavity of the insect host, from which

FlG. 124. — Plasmodium falciparum , the pernicious malaria parasite.

— From Muir and Ritchie.

A, Two small ring-shaped amoehulse within the corpuscles; B , a “crescent" 01
gamont, showing the envelope cf the red corpuscle. Figs. C-F illustrate the
changes in form undergone by the gamonts outside the body ; Fig. F shows a
male gamont that has undergone “ exflagellation,” or the formation of micro-
gametes. which are seen attached to it. (X 1000.)

13

194

MANUAL OF ELEMENTARY ZOOLOGY

the * sporozoites pass into the salivary glands. In these
the mosquito secretes a liquid which is injected into its
prey when it bites, and has the effect of stimulating the
blood-flow. When next it feeds, the little parasites pass
with the saliva along the proboscis into the blood of the
man on whom the mosquito is feeding, there to bore their
way into red corpuscles and start a new infection.

Long after apparent recovery from an attack of malaria,
a patient may suffer a recurrence of the disease. This is
probably due to persistence of a few of the trophozoites,
though it has been held to be caused by female gamonts
acting somewhat as the “ latent bodies ” of trypanosomes.

FlC. 125 — Part of the alimentary canal of a mosquito infested with
Plasmodium. — From Lankester’s Zoology , after Ross.

Zy., Cysts of the parasite ; mt., intestine ; A/.t Malpighian tubes ; ces ., oesophagus,

s/., stomach.

In comparing this life-history with that of Monocystis
some important differences appear, (a) In Monocystis the
trophozoite is not amoeboid, eventually outgrows its cell-
host, and does not reproduce asexually (i.e., does not be-
come a meront) ; (&) in the malaria parasite the gametes
are much more unlike than in Monocystis and the female
gamont gives rise only to one gamete ; (c) in Monocystis
there is no sporoblast generation between the zygote and
the sporozoites ; (d) in Monocystis there is probably no
necessity for a second kind of host, sexual reproduction and
the division of the zygote both take place in the host in
which the parent trophozoites live, and the zygote-cyst (the
pseudonavicella) does not burst and set free the sporozoites
until it reaches the host in which the new generation of

THE PROTOZOA AS PARASITES OF MAN

195

trophozoites are to live. The whole process is more highly
developed in the malaria parasite.

Malaria is very widely distributed. It is found in
tropical and subtropical lands of every quarter of the globe
and even in such temperate countries as England, where it
still lingers in marshy districts in the form of ague, once
much more prevalent than now. The parasites which
cause it are always transmitted by the Anopheline Flies
or true “ Mosquitoes,” being incapable of harbouring in
the related Culicine Flies, or “ Gnats ” in the strict sense,
which digest the gametocytes.

Gnats and mosquitoes, though they are much alike to
the untrained eye, may be distinguished by sundry small
differences, notably by the carriage of the body in the rest-
ing position (Fig. 126;. Both these kinds of flies lay their
eggs in water, and there, on hatching, the young pass
through larval and pupal stages (p. 345). It is practically
hopeless to attempt to destroy the adults on a large scale,
but they may often be prevented from breeding by doing
away with all suitable pieces of water, or attacking the
young stages by pouring paraffin over the breeding-places,
introducing fish which feed upon them, and so forth.

The loss of time, energy, and life itself from this disease
is very serious in many warm countries, notably, for
instance, on the West Coast of Africa. Formerly quinine
was almost the only resource against it, but the discovery
of the parasite and its life-history — a romance of Science,
in which the English observer Ross played a great part — has
made us less helpless to avert its ravages. The measures
which are taken to that end are threefold : (1) the destruc-
tion of the mosquito, by such measures as have been
mentioned ; (2) the use of quinine to overcome the

parasite in the blood and thus destroy the supply of it as
well as benefiting the individual patient ; (3) the separation
of European dwellings from those of the natives.

The Ciliata are not commonly parasites of man, but
one at least may cause in him serious disease,
coii?" Um Balantidium coli is related to Paramecium. It
has an egg-like body, about one-tenth of a
millimetre in length, with a funnel-like depression — the
peristome — at the narrow end, and a mouth and gullet at

MANUAL OF ELEMENTARY ZOOLOGY

196

the bottom of the peristome. A coat of cilia arranged in
parallel rows covers the body, and some larger cilia stand
beside the peristome. There is a good-sized, kidney-
shaped meganucleus, with a small, round micronucleus

Anopheles Culex

Fig. 1 2b — A comparison of the various stages in the life-history of a
mosquito (left) with those of a gnat (right). — From Shipley.

Note how the larvae and pupae hang from the surface film of the water (represented
by a thin line). The organs by which they are suspended contain air tubes, and
it these be prevented by a film of paraffin from functioning the insect is drowned.
Note also that the eggs of the gnat cling together as a raft.

e., two views of an egg, magnified.

against its hollow side, and two contractile vacuoles. Red
blood corpuscles are sometimes found in the food vacuoles.
Balantidium multiplies by transverse, binary fission, and
from time to time conjugates. In certain circumstances it
encysts. It lives in the large intestine of the pig, to which

THE PROTOZOA AS PARASITES OP MAN

107

it appears not to do much harm, and is spread by its cysts
passing out with the faeces, getting into water or on to
food, and being swallowed by a new host. When this is a
human being whose intestine is deranged, it increases the
irritation, penetrates the epithelium and lies in the layers
below it, and may even be carried by the blood or lymph
to other parts of the body and there cause abscesses. More
often it brings about dysentery, which may be fatal. It
occurs in all parts of the world. A Balantidium, is found,
with other ciliates, in the rectum of the frog.

The rectum of the frog contains an interesting population of ciliates,

which live chiefly in the lighter-coloured contents of

Oiliata of ■

the Frog.

pst.

■rrcex}

mi

C.Vf

its foremost region.

Balantidium entozoon
differs from B. coli in having four
contractile vacuoles and a longer
peristome. Ny ctotheru s cordi-
formis resembles the Balantidia in
its general features, but is bean-
shaped, with a long gullet placed
in the middle of the hollow side,
an undulating membrane, one con-
tractile vacuole in the hinder part
of the body, and a remarkable per-
manent anus, lined with ectoplasm,
at the hind end. The related N.
faba has been found in the in-
testine of a man suffering from
dysentery, but it is doubtful whether
it was the cause of the disease.

More numerous and conspicuous
than either of these is Opalina

ranarum, a flat, oval, pale-straw- Contraclile vacuoles t<xi

coloured ciliate of very large size vacuole ; g., gullet ; meg., mega-

(i mm. long), uniformly covered
with equal cilia, and without mouth,
peristome, contractile vacuole, or

trichocysts. It has many nuclei, unlike those of other Ciliata in
being all of one kind. The life-history is also very unlike that
of other Ciliata. Nuclei and cytoplasm divide independently (the
latter alternately in a longitudinal and a transverse direction),
and during the greater part of the year keep pace with one another
and with growth, so that the appearance of the mature animals
remains the same ; but in the spring the division of the cytoplasm
gains, so that small individuals with 3-6 nuclei result. It is said
that at this time a portion of the chromatin of the nuclei passes
in granular or “ chromidial ” form into the cytoplasm, where it
perishes. The little individuals now encyst. The cysts are passed

Fig. 1 27 . — Balantidium coll.

Contractile vacuoles ; f.v., io<
vacuole ; g., gullet ; meg., mega-
nucleus ; mi., micronucleus ; psl.,
peristome.

1 98

MANUAL OF ELEMENTARY ZOOLOGY

by the frog into the water and there swallowed by tadpoles, in which
they hatch and their cytoplasm divides to form uninuclear gametes,

Fig. 12S. — Ciliata from the rectum ot the Irog.

A, Opal ina ranarum ; B, Balantidium entozoon ; C, Nyctotherus cordifonnis.
an., Anus ; c.v ,, contractile vacuoles ; ec., ectoplasm ; en., endoplasm ; g., gullet ;
meg., meganucleus ; mi., micronucleus ; nu., nuclei ; pst., peristome.

the nuclei meanwhile undergoing a reducing division. The gametes
conjugate, and the zygote encysts. Probably it always at this stage

Fig. 129. — Opalina ranarum.

A, Ordinary individual. in longitudinal fission ; B, the same in transverse fission;
C, small encysted individual (distributive phase); D, gamete; E, encysted
zygote.

passes out of the host and enters another, where it hatches. From the
cyst emerges a uninuclear ciliate which grows into the adult. During

THE PROTOZOA AS PARASITES OP MAN

199

the whole process cilia are lost only in the zygote cyst. A comparison
of this life-history with that of Paramecium suggests (1) that the
destruction of some of the chromatin corresponds to that of the
meganucleus of Paramecium , (2) that the divisions of the micro-
nuclei of the latter before conjugation may perhaps be the remains
of a lost habit of forming a number of gametes like these of Opalina.

It does not appear that any of these animals is harmful to the frog.

Attention may now be called to certain processes in protozoan
Rioenine of nuclei which, doubtful of interpretation though they
Protozoan0 are none the less suggestive. Protozoa often (perhaps

Gametes, always) lose a portion of their nucleoplasm before

syngamy. This may take place by mitosis, and in such
cases a definite halving by the loss of chromosomes has sometimes
been discovered, as. in the micronucleus of Paramecium (p. 168), in
the nuclei of Opalina and in.those.of fhe Sporozoa and of Ch-lamydomonas.
In the latter two cases it happens in the first division of the zygote
so that the organism for the rest of its life has only a single set of
chromosomes. No doubt the function of such a process is the same
as that of the ripening of the gametes of higher animals (p. 136). In
other cases (Sporozoa, the chromidial process in Opalina) part of the
chromatic matter of the nucleus is lost by a non-mitotic process, and
this is perhaps comparable, not to the “ reduction division ” of Para-
mecium and the higher animals, but to the destruction of the body
nucleus (meganucleus) of Paramecium at conjugation, and thus, more
distantly, to the parting of body and germ in the Metazoa. It seems
that in Opalina and the Sporozoa both “ reduction ” and the loss of
body chromatin take place. Once more in these facts is probably to
be seen the analogy of the protozoon with the whole body of a meta-
zoon, rather than with one of its cells.

CHAPTER XI

SPONGES

That the objects of everyday use known as sponges are the
skeletons of marine animals is tolerably common
fiageiiata. knowledge ; but what may be the nature of the
soft parts of such animals is less well known.
A living sponge, however, is not only a very remarkable,
but even a fairly highly organised creature. Its nearest
relations, outside its own subkingdom, are to be found

among the Flagellate Protozoa.
Certain of these, known as the
Choanoflagellata , with a shape of
body not unlike that of Poly tom a,
but having like Trypanosoma a
single flagellum, are fastened by

FlG. 130. — Choanoflagellata.

a, Monosiga ; Salpingoeca ;
c, Polycecn.

Fig. 1 3 1 . - — Proterospong ia .

«, Nucleus of a collared member of the colony;
b, contractile vacuole ; c, amoeboid member ;
a, division of amoeboid member ; flagellate
member with collar contracted ; f, jelly ; g,
member forming spores.

200

SPONGES

201

,osc

-sp

a stalk at one end, and have at the other end the
flagellum, and around it a delicate collar ot protoplasm.
Fig. i so 'a shows one of these organisms. The Choano-
flagellata often secrete a cup or case which houses and
protects the body (Fig. 130, b)
and sometimes the cups form a
kind of colony, being joined into
a plant-like growth (Fig. 130, c) ;
but in one of them, known as
Proterospongia (Fig. 131), the
substance secreted takes the
form of a mass of jelly, in which
a number of separate individuals
are lodged. Some stand on the
outside, half-imbedded, with the
collared end projecting ; others
pass into the jelly and there
become amoeboid, and some of
these divide into minute spores,
which may be gametes. The
simplest sponge is a multi-
cellular organism, the elements
of whose body suggest a re-
arrangement of those of the
colony of Proterospongia.

This simple sponge is a little

creature, known as

1 8Smp*® the Olynthus, which
Sponge. s , Fig. 1^2 — The Olynthus

,s found only as a tlG's^e of a ca,/areous

fleeting stage in the develop-
ment of a few members of
the group ; but the bodies of

all may be regarded as ideally . .

derived from it, even tnougn.it ’fnwaii.

may not appear as a stage in

their life-history. It is a hollow vase, perforated by
many pores , and having at the summit a single large
opening, the osculum. Through the pores water con-
stantly enters it, to pass out through the osculum
Herein it and its kind differ from all other animals,
using the principal opening not for intaking as a mouth

stage of a calcareous
sponge, from which a
portion of the wall has
been removed to expose
the paragastcr.

202

MANUAL OF ELEMENTARY ZOOLOGY

— but for casting out. The wall of the vase consists
of two layers, a gastral layer , composed of collared
flagellate cells, or choanocytes, standing side by side but
not touching, which lines the internal cavity or paragaster
except for a short distance within the rim ; and a dermal
layer , which makes up the greater part of the thickness of
the wall and is turned in a little way at the rim. This layer
again consists of two parts, a covering layer of flattened
ceils, known as pinacocytes , rather like those of a pavement
epithelium, but with the power of changing their shape, and
the skeletogenous layer , between the covering layer and
the gastral layer. The skeletogenous layer consists of
scattered cells, with a jelly in which they are imbedded.

Fig. 133. — Part of a longitudinal section of the wall of an Olynthus,
including a portion of the rim of the osculum.

a. Amoeboid cell ; ch. , choanocyte ; e., flat covering cells (pinacocytes) of dermal
layer; similar cells lining the rim of the osculum; jelly; p., pore;
pc., young porocyte ; pc'., fully developed porocyte ; sp., spicule; sp.c.,
spicule cell.

The most numerous of these cells are engaged in secreting
spicules of calcium carbonate by which the wall is sup-
ported. They wander from the covering layer into the
jelly, and then each divides into two, and the resulting
pair secrete in their protoplasm, which is continuous, a
needledike spicule which presently outgrows them. Most
often the original spicule cells come together in threes
before this process, so that the three spicules which they
secrete become the rays of a three-rayed compound spicule.
This lies in the wall with two rays towards the osculum and
one away from it. Sometimes a fourth cell joins the others
later, and forms a fourth ray which projects inwards
towards the paragaster. Often there are simple spicules
which project from the surface of the sponge. Other cells,

SPONGES

203

known as porocytes , of a conical shape, extend through
the jelly, having their base in the covering layer while
their apex reaches the paragaster between the choanocytes.
Each is pierced from base to apex by a tube, which is one
of the pores. Besides these cells of the dermal layer there
are in the jelly wandering amoeboid cells which appear,
in some cases at least, to belong neither to the gastral nor
to the dermal layer, but to be descended independently from
blastomeres of the embryo. Some of them become ova ;
others, it is believed, give
rise to male gametes ;
the rest are occupied in
transporting nutriment
and excreta about the
sponge. The current
which flows through the
body is set up by the
working of the flagella
of the choanocytes. It
carries with it various
minute organisms which
serve the sponge for food,
being swallowed, in some
way which is still in dis-
pute, by the collar cells.

These digest the food,
rejecting the indigestible
parts into the space within
the collar ; and passing
on the digested food to amoebocytes, which visit them
to obtain it.

No sponge remains at this simple stage throughout
its life. At the least the body branches and
Sponge* thus complicates its shape* and then often

Borises. new oscula appear at the ends of the branches.

A higher grade is reached when, as in the
calcareous sponge Sycon , the greater part of the vase is
covered with blind, thimble-shaped outgrowths, regularly
arranged, and touching in places, but leaving between
them channels, known as inhalant canals , whose openings
on the surface of the sponge are often narrowed and are

Fig. 134. — A branched calcareous
sponge of the first grade. — From
Sedgwick.

MANUAL OF ELEMENTARY ZOOLOGY

20\

known as ostia. T he thimble-shaped chambers are known
as flagellated chambers , and are lined by choanocytes, but
these are now lacking from the paragaster, where they
are replaced bv pinacocytes. Water enters by the ostia,

os c

1 2

exh.c., Exhalant canal ; inh.c inhalant canal ; fl.c.y flagellated chamber ,
osc., osculum : ost ostium ; />., pore ; par.y paragaster.

passes along the inhalant canals and through the pores,
now known as prosopyles , into the excurrent canals, leaves
these through the openings, known as apopyles , by which
they communicate with the paragaster, and flows outwards
through the osculum. A third grade is found in sponges
•such as the calcareous sponge Leucilla , where the wall of

SPONGES

205

t he paragaster is folded a second time, so that the flagellated
chambers, instead of opening direct into the paragaster,
communicate with it by ex halant canals lined with pina-
cocytes.

The three grades of sponge structure, in which suc-
cessively the choanocytes line the whole paragaster, are
restricted to flagellated chambers, or are still further
removed by the presence of exhalant canals, are known
as the “ Ascon,” “ Sycon,” and “ Leucon ” grades. In
many of the sponges whose canal systems are of the third
grade, the flagellated chambers are no longer thimble-
shaped, but small and round. As the canal system has
grown more intricate,
complication has
taken place also in the
skeletogenous layer.

It has grown thicker,
forming outside the
flagellated chambers a
layer known as the
cortex , in which the
inhalant canals ramify ;
and there appear in it

branched connective T> .u c , - . -v

,, ... riG. 136. — i he Bain Sponge [Euspongia)^

USSUe cells which can — From Parker and Haswell.

change their shape.

The effect of these advances in complexity is (a) to increase
the rate and precision of the currents of water through the
sponge, (b) by strengthening the wall, to make it possible for
sponges to grow to a greater size.

The sponges of which we have so far spoken have
calcareous skeletons. A vast number of others
Siliceous have a skeleton of siliceous (flinty) spicules,

sponges™* In all these the canal systems are of one of the

more complicated types, and usually they are
made still more intricate by ramifyings of the paragaster,
and the appearance of numerous oscula, which put it into
communication with the water at many points. The
sponges of domestic use belong to a comparatively small
group in which the skeleton is not spicular but a network
of horny fibres, usually strengthened by sand grains

206

MANUAL OF ELEMENTARY ZOOLOGY

imbedded in the fibres. Their canal system is of the type
which has 'small, round chambers, and in most of them these
communicate with the exhalant canals by narrow aphodal
canals, as in the majority of the siliceous sponges. In
preparing the sponge for human use, the soft parts are
allowed to die and rot, leaving the horny skeleton, which is

Fig. 137. — A diagram of the structure of a bath sponge.

exh.c., Exhalant canal; rnh.c inhalant canal; flagellated chamber; esc.,

osculuin ; ost., ostium ; sd.c. , subdei inal ca\ ity ; sk. , one of the principal pillars of
the skeleton, containing imbedded sand grains ; sk’., minor fibres of the skeleton.

then cleaned. The large holes on the upper part of the
dried skeleton mark the position of the oscula ; in its in-
terstices formerly lay the ramifications of the canal- system. -
The softness and wearing qualities of the sponge depend
upon the fineness of the meshwork of its skeleton, and upon
the amount' of the sand particles which are embedded in it.
The true Ba.th Sponge (Etispongia officinalis') has very few
foreign particles. It is gathered in 10 to 15 fathoms of

SPONGES

207

water, the finest varieties from the Adriatic, coarser ones
from elsewhere in the Mediterranean, the West Indies, and
Australian seas. Various species of Hippospongia yield
coarse kinds of sponge.

Sponges have free larvae, of several different kinds, but
all covered with flagellate cells, by which they swim. The
remarkable feature of the metamorphoses by which these
larvae become the fixed adults is that the flagellated cells
pass into the interior, develop collars, and become the
choanocytes.

The sponges are known in Zoology as Porifera . In
PoHtera. their bodies consist of many “ cells,” they

might seem to be Metazoa. But they differ
from all members of that group in several important
respects. In no metazoan are choanocytes found. In
none is the principal opening exhalant. In none is there
during development an inversion whereby a flagellated
outer covering becomes internal Lastly, and perhaps
most significantly, in a sponge the “ cells ” are far less
specialised and dependent upon one another than the cells
of a metazoan. Many of them can assume various forms,
becoming amoeboid, collared, etc. Many are isolated in
the jelly, and when they touch they are not continuous.
There is no nervous system. Even the choanocytes,
though their efforts together produce a current, do not
keep time in their working. In short, the Porifera are
practically colonies of Protozoa. For these reasons it is
best that, in a classification of animals, they should be
given, under the name of Parazoa, the same rank as the
Protozoa and Metazoa.

CHAPTER XII

HYDRA. POLYPS AND MEDUSAE.

CCELENTER AT A.

If a handful of weeds gathered from a freshwater pond
be placed in a beaker of water and allowed
Hydra : to stand for a while, there will often be found

Features. hanging from the sides of the beaker or from
the weeds some short threads of a green,
brown, or whitish colour. By one end each thread cleaves
to the glass. At the other it bears about half a dozen
finer threads, which hang down in the water if they be left
undisturbed. A touch will cause these to be withdrawn
and take on a shorter and thicker shape, interference with
the thread from which they hang is followed by a similar
change, and in this way the whole can be made to contract
into a vase-shaped mass surmounted by a circlet of little
knobs. From time to time water-fleas and other small
animals swim against the fine threads and may be seen
either to drop through the water as though they were
stunned, but afterwards to recover and swim away, or else
to remain sticking to the fine threads, which shorten and
draw the animal towards the end of the main thread, into
which they are swallowed. It is dear that these objects are
living beings: in point of fact each of them is a specimen
of the animal known as Hydra . According to their colour
they hawe been named H. viridis , H. fusca, and H. gnsea.
The three kinds differ slightly in other respects besides
colour, but the following account applies to all of them.

The body of Hydra is a hollow cylinder, with a ring of
hollow outgrowths or tentacles surrounding an
snap®.. opening or mouth at one end, and the other

end closed by a flat basal disc or foot. The mouth is raised

2ut>

HYDRA. POLYPS AND MEDUS/E

■log

upon an oral cone or hypostome ; it leads into the hollow of
the cylinder, with which the hollows of the tentacles are
continuous. This space is the enteron. The cylinder is
rather wider in the middle than near the ends. The wall of
the body is composed of two protoplasmic layers, the outer
known as the ectoderm and the inner as the endoderm , with

a structureless la-
mella or mesoglea
between them, con-
sisting of a gelatin-
ous substance which
they secrete. Such
a body as this is
known as a polyp.

The ectoderm

_ , _ consists

Ectoderm. r

ot sev-
eral kinds of cells, of
which the most con-
spicuous are those
known as musculo-
or myo - epithelial
cells. These have
broad outer ends,
which meet and form
the surface of the
body, standing on
several pillars which
T7> q -r • c tt 7 • reach and expand

riG. 130. — two specimens or Hydra magm- A ■,

fied, one contracted, the other in a state uporL the mesoglea,
of moderate expansion, the latter bearing where each cell is
two buds in different stages. drawn OUt into One

m., Mouth ; or. c., oral cone. t)I more Contractile

processes. The
processes, each containing a fibre, run along the cylinder
and tentacles, at right angles to the cell, forming a distinct
layer on the outer side of the structureless lamella. Over
the greater part of the body the surface layer of the proto-
plasm is a firm pellicle, but in the disc this is absent. The
cells in this region are also peculiar in containing granules of
a substance secreted by the protoplasm which is used to fix the

14

210

MANUAL OF ELEMENTARY ZOOLOGY

animal to the surface it
hangs from. Each mus-
culo-epithelial cell has
a large oval nucleus in
one of its pillars. In the
tentacles these cells are
less tall than elsewhere.
Between the pillars are
spaces which contain
small, rounded inter-
stitial cells. These form
a reserve from which,
in various circumstances,
any of the other ceils of
the body can arise. Thus
they retain the undiffer-
entiated nature of the
germs and are sometimes
called indifferent cells.
Between the pillars
stand also peculiar cells
known as cnidoblasts ,
which project through
the surface protoplasm.
These are very numerous
in the tentacles, where
they lie in groups or
batteries (Figs. 139, 140),
but absent from the basal
disc. Each of them has
a pear-shaped body with
the narrow end at the
surface of the animal,
where there projects
from it a short process
known as the cnidocil .
On this side the cell
contains a pear-shaped
sac, called the nematocyst.
The narrow outer end of
the sac is tucked in and

Fig. 139. —A diagrammatic, longitudinal
section o {Hydra, magnified. — From
Shipley and MacBride.

bat., Battery of nematocysts. Only a few of
these axe shown ; they cover the tentacles ;
ect., ectoderm; end., endoderm ; ent., en*
teron ; /., foot ; h t., hoi low of a tentacle ; ov.,
ovary; st.l. , struc'ureless lamella; t. testis.

HYDRA

211

Fig. 140. — Portions of tentacles of Hydra magnified.

A, Moderately contracted ; B, moderately extended.

bat., Batteries; cnc., cnidocil ; ect ., ectoderm; end., endoderm ; ntc., nentato-
cyst-.

Some of the batteries, and parts of others, are seen through the thickness of the

tentacle.

Fig. 141. — Hydra in the attitudes which it assumes successively in
two of its modes of locomotion.

A, “Looping ” ; B, “ somersaulting."

2 I 2

MANUAL OF ELEMENTARY ZOOLOGY

produced into a long, hollow thread, which lies coiled up in
the sac. The space between the thread and the wall of the
sac contains a fluid. The cnidocil is a sense organ. When
it is stimulated the thread is expelled, being turned inside
out in the process. It is said that this is brought about by.
pressure which the shortening of contractile fibres in the
protoplasm around the sac exerts the fluid in the latter.1
The nematocysts are of three kinds — a large kind with a
straight thread provided with barbs at the base, a small
kind with a spiral thread, and a second small kind with a

straight thread and a nar-
rower sac than the others.
Neither of the small kinds has
barbs. The broad end of
each cnidoblast is anchored
into the body by a process
which runs inward to the
structureless lamella. The
tentacles are covered with a
number of warts, each con-
sisting of a large muscuk>
epithelial cell, in which is
sunk a battery of cnidoblasts
consisting of one or two of the
Fig. 142. — A transverse section of large kind with several of the
Hydra, stained and seen under smaHer kinds around them,
thejow power of the mtcro- Each Qf ^ k;nds of nemat0.

cysts has a functionof its own.
Those of the large, barbed
variety are weapons of offence
and perhaps also of defence. The sensitiveness of their
cnidocils to tactile stimuli is increased by chemical sub-
stances, given off from the bodies of other animals. When
the nematocysts are discharged, their barbs emerge first
and make a wound in the tissues of the prey, into which the
thread is driven. In piercing the horny skin of the water-
fleas, upon which the Hydra principally feeds, they are
assisted by the corrosive action of a fluid which they con-
tain, either in the hollow of the tucked-in thread or in that

1 Probably the extension of the thread is continued by the action of
a mechanism in the thread itself.

scope

eel.. Ectoderm ; end., endoderm ; st.l.,
structureless lamella.

HYDRA

21

of the sac. This fluid also temporarily numbs the prey, but
the main function of the nematocysts is not to kill but to

The figures are diagrammatic and not drawn to scale.

A, Musculo-epithelial cells ; B, a nerve cell ; C, a cnidoblast ;

D , nematocysts of three kinds ; E, Zoochlorelke ; F , a sense cell.

cnb Cnidoblast ; mb' , interstitial cell which will become a cnidoblast, with vacuole
for nematoevst ; cnc., cnidocil ; fib., fibre ; int.c., interstitial cell ; m.prs., con-
tractile process ; m.s.c., musculo-epithelial cell ; n.c., nerve cell ; ntc., nemato-
cyst ; nu., nucleus ; prs., basal process of the cnidoblast ; s.c., sense cell.

hold the prey until it is swallowed. In this the spiral kind
assist by coiling round bristles upon the body ol the prey.

MANUAL OF ELEMENTARY ZOOLOGY

214

int.c.

ntc.

The third kind of nematocysts is of use in attaching the
tentacles of the animal, either to its prey or to other objects
when necessary, by the stickiness of the threads. The
cnidoblasts arise from the interstitial cells by the formation
of a vacuole and its gradual modification into a nema-
tocyst. They are formed in the upper region of the cylinder

and migrate thence to various
parts of the body, where they
take up their position in the outer
layer. The germ cells also arise
in the ectoderm from the inter-
stitial cells by a process which
we shall describe later. Lastly,
among the bases of the ectoderm
pillars lies a mesh-work of
branching nerve cells which is
joined by rootlets from tall,
narrow sense cells that, like the
cnidoblasts, pierce the surface
protoplasm. How these cells are
connected is not certain ; prob-
ably it is not by continuity of
their processes but, as in the
synapses of higher animals (p. 94),
~A sma11 portion py contiguity. Physiologically

these connections differ from such
synapses in that impulses can pass
either way across them. The sense
cells are not specialised to serve
particular senses. Thus Hydra
possesses a nervous system, but
this is in the most rudimentary
condition possible, being a mere
net of cells conducting in all
directions, without nerves or central nervous exchange such
as the frog has, while the cells from whose bodies the afferent

s/. p-

Fig. 144

of a
of Hydra.

ect.y Ectoderm ; end., endoderm ;
f.p., food particle, ingested

by an endoderm cell ; int.c.,
interstitial cells ; m.e.c.,

musculo - epithelial cell ;

ntc., nematocyst ; „ st.l.,

structureless lamella ; vac.,
vacuoles in endoderm cells ;
vac'., vacuoles in ectoderm
cells.

fibres arise are among those which form the surface layer,
as in the olfactory epithelium of the frog, not removed
from it like those of most of the afferent fibres in the latter
animal (Fig. 61). The nervous system governs only the
muscles. The cnidoblasts are independent of it.

HYDRA

215

Such a nervous system is known as a nerve net. It is at
first hard to see how with it any variety of
the^Nerve^iyet. acti°n is possible. It would seem that the
only result of any stimulus to it must be a
general contraction of the body. Actually, however, the
behaviour of Hydra , though it is much simpler than that
of higher animals such as the frog, is, as we shall see,
fairly complicated, and shows a good deal of adaptation
to circumstances. This, of course, means contractions
which are local and vary with conditions. It is due
to several factors, some of which we have already met
m considering the nervous system of the frog (p. 94).
(1) The stimuli which the system receives vary in strength,
and the stronger the stimulus the more effective are the
impulses it starts. (2) Owing to the numerous synapses
between the many cells of the net, the impulses become less
effective as they go, so that those set up by strong stimuli
travel farther and set in action a larger part of the
body than those of weaker stimuli. (3) Stimuli which
are too weak to be effective singly may be summated,
so that a small stimulus repeated will have effect where
at first it has none. (4) The activity of one part of the
body may set up impulses which affect others. Thus the
activity of one tentacle brings others to its aid, and
the tentacles set the mouth in action. (5) The reaction
both of the receptors (sense cells) and of the effectors
(muscle fibres) varies from time to time, as with hunger
or fatigue. (6) Possibly, as in many other animals, some
of the muscle fibres respond more easily than others to
stimuli. Inhibitions (p. 95) probably do not occur in the
nerve net. By receiving a stimulus, one part of the body
becomes dominant and the messages started in it set other
parts in action. That is why the body behaves as a whole.
That its action is adjusted to particular stimuli is due to
the factors just stated.

It is instructive to contrast this nervous system with that
of the frog (or any other animal higher than the coelenterata
— the group to which Hydra belongs). There each nerve
cell has a long process — a nerve fibre — and such processes
are gathered into trunks — nerves — which run to and from
a central exchange, and, instead of being broadcast as in

216

MANUAL OF ELEMENTARY ZOOLOGY

Hydra , the impulses are directed to particular organs.
The advantages which such a system has over the nerve
net are great. It requires for a given effect fewer nerve
cells and so avoids the weakening of impulses at numerous
synapses, is more precise, and gives better facilities for the
co-ordination of action in distant parts of the body. We

Fig. 145. — A Diagram of the Nervous System of Hydra . — After

Iiadzi.

shall see the beginning of such a system in the Flat-
worms (p. 240).

In the endoderm the cells are tall and columnar.

Some of them, especially numerous in the
oral cone and absent from the tentacles, are
glandular . They have a narrow stem and a wide end,
turned towards the enteron and containing granules of a

HYDRA

217

•substance which they secrete. The most numerous and
conspicuous cells are nutritive. They are stout, and have
their bases produced into contractile fibres, which are
shorter than those of the musculo-epithelial cells and run
around the body, not along it. Their protoplasm contains
large vacuoles, and also, in the green Hydra , a number
of round bodies of a green colour, each of which consists
of a central mass of protoplasm with a covering of a
different kind of protoplasm containing the green substance
known as chlorophyll , to which the colour of plants is due.
These bodies multiply by division. In the brown Hydra
the green bodies are absent, but there are present some
yellowish bodies of similar shape, in which, however, no
structure can be made out. The ends of the cells which
abut on the enteron bear flagella, which can be replaced
by pseudopodia. There are some sense cells and a few
nerve cells.

The green bodies of Hydra viridis are degenerate, non-
flagellate individuals of a minute plant, related to Chlatny -
domonas , and are known as Zoochlorellce . Like other green
plants they nourish themselves by building up complex
organic compounds from simple inorganic ones (carbon
dioxide, water, salts, etc., see p. 26). They obtain these
simple substances as waste products of the metabolism of
the Hydra. It may be that the Hydra absorbs from them
in return the excess of carbohydrates which they form ;
and this would account for the absence from them of
starch, which is so constantly found in plants. Thus there
is between the two organisms a partnership, in which the
animal benefits by the removal of waste products and the
supply of oxygen and possibly of carbohydrates, and the
plant benefits by the rich supply of nitrogenous material and
carbon dioxide. Such a partnership is known as symbiosis
and is in contrast with parasitism, in which one of the
partners benefits at the expense of the other.

The movements of Hydra are carried out mainly by the
muscular processes of the cells, though the
and Reactfons. surface of the basal disc can put forth pseudo-
podia, and it is possible that by means of these
the animal can slowly change its position. The muscular
processes of the ectoderm cells, when they contract, make

218 manual of elementary zoology

the body shorter and wider ; those of the endoderm make
it narrower and longer. The position of rest is one of
moderate extension. Hydra does not remain passive in
the absence of stimuli, but, after standing for some time
extended in readiness for prey, it automatically contracts
either the whole body or the tentacles only, and then extends
in a new direction. Thus it explores the whole of its sur-
roundings. From time to time it changes its position.
This is done by extending the body and bending it, so that
the tentacles touch some neighbouring object and adhere to
it by means of the nematocysts with sticky threads. The
basal disc is then either withdrawn altogether from the spot
to which it was fixed and put down in a new spot close to
the tentacles, or caused to glide up to the tentacles. In either
case the animal moves in somewhat the same way as a looper
caterpillar (Fig. 141). A Hydra responds to every stimulus,
except that of food, by contracting. If the stimulus be weak
it affects only the part of the body to which it is applied,
as a single tentacle will withdraw from a slight touch ; if it
be strong its effect spreads to the whole body. A stimulus
applied to one side of the body a number of times causes it
presently to move away in some other direction. Hydra
avoids both too feeble and too strong a light.

The food of Hydra consists of water-fleas and other
small animals. These are caught by the
Exoreion.and tentacles, and carried by them to the mouth,
which then opens and swallows the prey. The
animal will not feed unless it be hungry. If it be
well fed, creatures which swim against the tentacles are
allowed to escape, but, if food has been scarce, as soon as
the prey has become temporarily attached by the nemato-
cysts to one tentacle the others bend over towards it and
help to secure it and push it towards the mouth. If the
animal be starving the mere smell of food in the neighbour-
hood is enough to set the tentacles working, but usually
they are not put into action till the food has been both
smelt and touched. It is not possible to deceive the
Hydra into swallowing substances, such as pieces of blotting-
paper, which do not smell like food, but blotting-paper
soaked in beef-tea is swallowed when it touches the tentacles.
Once swallowed, the food is passed deep into the enteron

HYDRA

219

and there softened by a juice which the gland cells secrete,
broken up by the churning which it gets as the body ex-
pands and contracts, and swept about by the flagella.
Part of the food is dissolved in the enteron and absorbed
in solution, part of it is taken up by pseudopodia of the
endoderm cells and digested within their protoplasm.
Presumably the ectoderm is nourished by substances passed
on from the endoderm, either by diffusion through the
structureless lamella or along the fine threads of protoplasm
which put the two layers into connection across it. The un-
digested remains of the food are driven out of the mouth by
a sudden contraction of the wall of the body. In unnatural
conditions of culture the animals become liable to depres-
sion much like that of Paramecium , in which the powers of
movement, feeding, and fission are affected and death
ensues. Respiration and excretion probably take place
from the surface of the ectoderm and endoderm ; there is
no special organ for either process.

The species of Hydra reproduce themselves both

sexually and asexually. The sexual reproduc-
Reproduction. t*Qn v^r^s anci // grisea takes place

normally in the spring and summer, that of H.fusca in the
autumn. The animals are usually hermaphrodite, but strains
are met with in which the sexes are separate. The genera-
tive organs are ectodermal structures developed when
sexual reproduction is about to take place. The ovaries,
of which there is generally only one in each individual, are
found in the lower part of the body ; the testes, of which
there are several, are in the upper part. In the early
stages of both organs the interstitial cells multiply and push
out the musculo-epithelial cells so as to form a swelling. In
the case of the ovary one of the interstitial cells becomes an
oocyte (p. 1 31). This increases in size and begins to throw
out pseudopodia, by which it swallows the rest of the inter-
stitial cells contained in the swelling. At the same time it
lays up in its protoplasm numerous dark, spherical granules
of yolk. As the swelling increases, the musculo-epithelial
cells are stretched, their conical bodies forming long stalks,
which are pushed apart by the oocyte, and their outer layer
forming a thin covering for the latter. When the oocyte has
swallowed all the surrounding cells it withdraws its pseudo-

220

MANUAL OF ELEMENTARY ZOOLOGY

podia and becomes a large rounded body, about which a
gelatinous coat is secreted. Polar bodies are now formed,
the covering of musculo-epithelial cells parts and shrinks
back so that the. ovum is exposed save for the gelatinous
coat, and fertilisation is effected by one of the spermatozoa
which are present in the surrounding
water. In the formation of a testis
the multiplication of the interstitial
cells stretches the musculo-epithelial
cells as in the ovary. The interstitial
cells become spermatocytes, which
lie among the stalks of the musculo-
epithelial cells and undergo two
divisions as in the frog, the resulting
cells developing into spermatozoa
with a conical head, a neck, and a
tail. By the breaking of the cover-

ing layer the spermatozoa are set lUhomAtTJZT
free and swim in the water, where
they perish unless they find a ripe
ovum. Since either the ovary or the
testis generally ripens first, cross-
fertilisation will usually take place, but it does not ap
pear that self-fertilisation is always impossible.

ov.

Ovary, with nearly ripe
ovum ; te., testes.

See also Fig. 147.

After fertilisation the oosperm undergoes cleavage into blastomeres
Development I37)> which as they increase in numbers form at first
a hollow sphere known as the blastula, whose wall con-
sists of a single layer of cells. Some of these migrate into the hollow
which they fill. The outer layer now represents ectoderm and the
inner mass endoderm. The cells of the ectoderm become smaller than
those of the endoderm and lose their yolk granules. A thick, spiny
covering of a horny substance is now secreted by the ectoderm, and
the round, prickly body thus formed falls away from the parent and
rests for several weeks, during which it may be carried about by
cunents, in mud on the feet of water animals, etc. After a time the
ectoderm differentiates into musculo-epithelial and interstitial cells,
the jelly is secreted, the shell cracks, and the embryo projects. A
split in the endoderm forms the enteron, tentacles grow out, a mouth
is formed, and finally the young Hydra frees itself from the remains
of the shell, moves away, and begins to feed and grow.

Asexual reproduction also begins with the formation of a
swelling of ectoderm by the multiplication of the interstitial
cells, which afterwards become converted into musculo-

HYDRA

221

epithelial and endoderm cells, passing through the structure-
less lamella in the latter case. The result of this is an

<

increase in the extent of the ectoderm and endoderm which
leads to a bulging of the body wall. The knob or bud thus
formed becomes longer, tentacles grow out around its free

Fig. 147 — Reproductive organs of the green Hydra .

In each case a testis is shown above, to the left, and an ovary below, to the right. In A the ovum is unripe,
in B it is ripe, has burst its covering of ectoderm cells, and hangs by a stalk. The large round spots in
the ovum are Zoochlorella.

222

MANUAL OF ELEMENTARY ZOOLOGY

end, a mouth is iormed, and finally the base narrows till the
bud breaks free as a new individual, which grows till it
reaches the size of the parent. The buds arise in the middle
of the body of the parent. Several may be formed at the
same time, and a bud may form secondary buds before it is
set free. While it is still on the parent, the bud is wholly a
part ol the body of the latter. Each of the layers of the

Fig. 146. The development cf Hydra. — After Brauer.

1. sp., Spermatozoa.

2. Amoeboid ovum ; g.v., germinal vesicle or nucleus ; v s yolk

spherules. ^

3. Ovum protruding; »., its nucleus; eel., the ruptured ectoderm of

the parent ; end., the endoderm.

4. Prickly envelope (sh.) of embryo liberated from parent.

5. Section of blastula — Ect., ectoderm ; End., endoderm arising ; it will

fill the blastula cavity (blastocoele).

6. Section of young Hydra leaving shell. Ect., ectoderm ; End., endo-

derm : g.c., enteron ; sh., ruptured envelopes.

parent is continuous with the corresponding layer of the
bud, a suitable stimulus is transmitted by the nervous
system from one to the other, and the entera are in free
communication, so that food obtained by either is available
for the other. Occasionally a Hydra will reproduce by
fission into two, either lengthwise or transversely, of the
whole body. In this case, as in the fission of a Paramecium.
structural development as well as the growth of each pro-
duct cf fission must take place after separation, whereas in

HYDRA. POLYPS AND MEDUSAE

223

the bud, as we have seen, the structural development takes
place before fission.

A property akin to asexual reproduction is that of
Regeneration, regeneration or the replacement of lost parts,
which is possessed by Hydra in a very high
degree. To some extent all organisms have this power,
but as a rule the higher the animal the less is its faculty for
regeneration. In man it is little more than the power of
healing wounds. Not only will Hydra grow anew any part,
such as a tentacle, which is cut off, but any fragment of the
body, provided it be not too small and contain portions of
both layers, will grow into an entire animal.

We must now iook at the budding of Hydra from a sorrie-
what different point of view. By the out-
Coicnies. growth of buds, the animal increases the size of
its body in precisely the same way as Carchesium ;
that is to say by the addition of new members, each of
which repeats the whole structure of the body as it existed
at first. In the case of Hydra the process is carried further
by the fission of the repeated part from the parent body, so
that an act of reproduction takes place, but it is easy to
imagine a case in which this would not happen. The result
would be the permanent conversion of the body of the
Hydra into a colony, of which the buds would be the
zooids. Now there are a number of animals related to
Hydra in which this actually takes place. Such animals
are known as hy droids, and nearly all of them are marine.
A common example is Obelia geniculata , which is found
growing upon seaweeds near low watermark on the British
coast.

Certain comparatively unimportant differences distin-
guish the polyps of Obelia from those of Hydra.
Anatomy of tentacles are more numerous and, instead

th« Polyp. of being hollow, have a solid core of large
endoderm cells, with very stout walls of inter-
cellular substance and highly vacuolated contents. In the
ectoderm the muscular fibres are independent cells with
nuclei of their own, lying below the epithelium. The oral
cone is very large and forms a chamber above the rest of
the enteron. From the middle of the basal disc of each

224

MANUAL OF ELEMENTARY ZOOLOGY

polyp the body-wall is continued as a narrow tube, which
joins the tubes from other polyps so as to form a branching
structure like the body of a flowering plant. This is con-
tinuous at its base with a root-like arrangement of tubes on
the surface of the seaweed, known as the hydrorhiza. The
tubes of the whole structure are known as the coenosarc , and
the polyp heads as hydranths. The whole colony is
enclosed in a horny case or perisarc , which is secreted by

the ectoderm and follows
closely the outline of the
body, but is separated from
it by a small space, bridged
by processes from some of
the ectoderm cells. At the
base of each hydranth the
perisarc expands into a cup
or hydrotheca into which the
hydranth can be withdrawn.
The generative organs are
.. j not borne by the
polyps, but by
special bodies, which origin-
ate as members of the colony,
are set free by breaking away
as the buds of Hydra are, and
carry out sexual reproduction
as independent individuals.
These individuals differ
widely from the polyps, be-
ing, indeed, so unlike them
that their origin from the
colony would never have been
guessed unless it had been seen to take place. They are
small jelly-fish or medusce. Each has the shape of an
umbrella with a short, thick handle and a fringe of tentacles
around the edge. The convex upper side is called the
exumbrella, the concave lower side the subumbrella , the
handle the manubrium. Around the edge of the umbrella
a low ridge projects inwards. This is the velum and repre-
sents a much larger structure in the same region of many
other medusae. At the end of the manubrium is the mouth,

POLYPS AND MEDUSAE

22

Fig. 15°. — Part of a colony of Obelia, magnified.

bl. , Blastostyle ; ect., ectoderm ; end., endoderm; gth., gonotheca ; hylh ,
hydrotheca; med., medusa bud; p.b., polyp bud; perith., peritheca.

15

to

226

MANUAL OF ELEMENTARY ZOOLOGY

-end. t.

--ect.

---it- end.

which leads by a tubular gullet along the manubrium to a
stomach in the middle of the body. From this four radial
canals run outwards to a ring canal at the edge of the um-
brella. The lining of all these internal spaces consists of
endoderm, and the radial canals lie in a sheet of endoderm,
known as the endoderm lamella. In fact we may regard

the internal cavi-
ties of a medusa
as corresponding
to the enteron of
a polyp in which
the walls have
come together
over a large area,
leaving certain
spaces which
form the gullet,
stomach, and
canals. The
whole outside of
the body and ten-
tacles is covered
with ectoderm.
Between the ecto-
derm and the
endoderm is a
layer of jelly,
which is very
thick, especially
on the exumbrella
side. The medusa
may be compared
to a polyp which is greatly widened and shortened, the
walls of the wide, flat enteron coming together in places, as
we have seen, and the structureless lamella increasing in
thickness to form the jelly. The manubrium represents the
oral cone and the tentacles stand around it at a greater
distance owing to the widening of the body. The arrange-
ment of the organs of a medusa is an excellent example of
what is known as radial symmetry. In bilateral symmetry
(p. 33) the parts of the body are arranged on each side

Fig. i 5 i.-— A longitudinal section of a hydranth
of Obelia, highly magnified.

ect.. Ectoderm ; end., endoderm ; end.t., endoderm of the
tentacles ; hyth., hydrotheca ; or.c., oral cone ; st.l.,
structureless lamella.

POLYPS AND MEDUSAE

227

(right and left) of a plane, in such a way that no other plane
will divide the body into two halves which are alike. In

Fig. 152. — A medusa of Obelia, magnified.

radial symmetry the parts of the body are arranged about a
point in such a way that innumerable planes divide the
body into like halves. Polyps are also radially symmetrical.

, vnt.

A Be

Fig. 153. — A diagram to illustrate the relation between polyp

and medusa.

A, The polyp ; B, an imaginary intermediate form ; C, the medusa.

can. c. , Circular canal; can.r ., radial canal ; ect.} ectoderm \ end., endoderm ; ent.,
enteron ; m., mouth ; mb.y manubrium ; or.c ., oral cone ; ten ., tentacle ; vm. ,
velum. The dotted line represents the velum as it is found in many medusae
but not in Ubelia.

The medusa floats in the sea with the manubrium down-
wards and the tentacles hanging like the
th0VMedu8a.°f snaky locks of its classical namesake. It swims
by contractions of the plentiful musculature of
the subumbrella side, which drive water out of the umbrella
and send the animal forwards in the opposite direction.

228 MANUAL OF ELEMENTARY ZOOLOGY

gives

rise

--■5

I he contractions are started by impulses which originate
in the nerve net at the umbrella margin. There nervous
transmission is facilitated by the nerve-rings — two specially
well-developed circular tracts of the net — and there is
provision for keeping balance by means of eight sense organs,
known as statocysts , situated each at the base of one of the
tentacles. These are small hollow vesicles each containing
a calcareous body which hangs in a single cell that secreted
it. The swaying of the calcareous bodies against fine pro-
cesses on sense-cells which line the outer side of the vesicle
to impulses by which the movements of the

animal are regulated
through the nervous system,
stronger contractions being
causedon the side which dips .

The medusae are of oppo-
„ J , . site sexes. The

Reproduction

generative or-
gans are not developed till
after the animal is set free.
They are four in number and
lie on the subumbrella below
the radial canals. Each con-
Fig. 154.— The medusa of Obelia, sists of a knob of ectoderm

seen from the subuml.rella side. jnto which passes a short
-From Shipley and MacBnde. branch fromfhe radiaIcana,

The germ mother - cells
originate in the ectoderm of
the manubrium, migrate into
the endoderm, and pass along the radial canals to the gener-
ative organs, where they migrate into the ectoderm again.
When the ova or sperm are ripe, they are shed by the rupture
of the ectoderm into the water, where fertilisation takes
place. As in Hydra , segmentation leads to the formation
of a hollow blastula. From this, by immigration of cells
at one spot, there is reached a stage with a solid mass of
endoderm such as that found in Hydra. The animal at
this stage is of a lengthened egg-shape and has a ciliated
ectoderm, by which it swims freely for a while. It is known
as a planula. The planula then settles down by its broader
end, an enteron is formed by a split in the endoderm,.

Mouth, at end of manubrium; 2, ten-
tacle ; 3, gonad ; 4, radial canal ; 5,
statocyst.

POLYPS AND MEDUSAE

229

tentacles and a mouth form at the other end, and
there develops a

J

. I ,1 L

St

ex. it.

canr.

can.c.

polyp, from which
by budding a
colony arises.

When the colony
has reached a
certain size there
appear, in the
angles between
the stem and the

branches which Fig. 155.— a diagiam of a vertical section of
bear the hydranths, the1 medusa of Obelia. The section is

tubular out- supposed to pass on one side along a

crrnwrhc trmum « radial canal, and on the other across the

7. ; ,° 11 'S endoderm lamella. In reality this would

lastosty/es, each not be possible, since the canals are

enclosed in a vase opposite one another.

of perisarc known
as a go no thee a. A
blastostyle and its
gonotheca are to-
gether known as a

can.c ., Circular canal; can.-r., radial canal; tn.l.,
endoderm lamella; car.u., exumbrella surface; g.,
gonad ; j., jelly ; m., mouth ; mb., manubrium ;
n.r., nerve ring; ass., cesophagus; s.u., subum-
brella surface; st., stomach; sic., statocyst ;
icn tentacle; vtn., velum.

gonangium. The blastostyle

Fig. 156. — A diagram to show the
development of medusae as
buds on a blastostyle.

bl., blastostyle ; s.u.p., subumbrellar

cavity ; 1-6, successive stages in the
development of a medusa.

is probably an incomplete
zooid. On its sides are
formed a number of buds
which develop into little
medusae and escape through
the opening at the top of the
gonotheca.

It will be seen that Obelia ,
like Hydra , re-

Generations.0* produces itself
both sexually
and a sexually. Sexual re-
production is carried out by
the medusa and leads to the
formation of polyps. The
asexual reproduction con-
sists in the budding off of
medusae from the polyp
stock. Whereas, however,

230

MANUAL OF ELEMENTARY ZOOLOGY

in Hydra , the two processes go on side by side, sometimes
in the same individual, and succeed one another quite
irregularly, in Obelia there are two different types of
individual — the polyp stock and the medusa — which follow
one another regularly and are each confined to one method
of reproduction. Thus wg have a definite alternation of
generations , a sexual and an asexual form succeeding one
another. It will be remembered that such generations
also alternate in Mondcystis ancf that the malaria parasite
has a more complicated life-history of the same kind.
The asexual generation of Obelia is relatively inactive,
gathering much nourishment and spending little : the sexual

generation is active, spending
its substance freely in locomo-
tion, which ensures the dis-
tribution of the species and
thus opens up fresh food
supplies and increases the
chances of escape from local
dangers. The gist of the
story is the distribution of
labouT among individuals of
different kinds.

A B

flG. 157. — A , Planula larva;
B , the young polyp into
which the planula grows
after settling.

The designation

Metagenesis.

ot generations

“ alternation
has

been applied to a
number of different types of life-
history which have in common only the fact that reproduction is
accomplished differently in successive phases of the reproducing
organism. It is a useful “ omnibus ” term but should not be taken

to imply more than superficial resemblance between the processes it
earns. The type met with in hydroids is known as Metagenesis.
We shall observe a quite different process, which does not involve
asexual reproduction, in the Liver Fluke and again in a nematode
worm. Yet another is seen in plants, very clearly, for instance, in
ferns. That in the Sporozoa has (in respect of chromosome sets)
some resemblance to that of plants. In view of this diversity, many
biologists prefer not to speak of the life-history of Obelia as an
alternation of generations, but to use for it the term metagenesis.

The above account of the reproduction of hydroids
differs in one respect from that which is generally given.
On the analogy of the budding of Hydra , it is usual to
regard the formation of a hydroid colony by budding as a

POLYPS AND MEDUSAE

231

kind of asexual reproduction in which there are formed
numerous “ individuals ” which do not separate.
Reproduction In that case the alternation of generations
bufiding0ny* contains an indefinite number of acts of
asexual reproduction between each two sexual
acts. We have preferred to treat the polyp stock as
one individual containing a number of semi-independent
parts — the hydranths — each of which repeats the structure
of the whole body as it was at first, and having certain
other parts — the blastostyles and most of the ccenosarc
— which are differently constructed, serve the entire
body, and are wholly dependent upon it. This view in-
volves the following considerations. The development
ot the individual and its reproduction are essentially the
same process — - morphogenesis , which is also at work in
regeneration. Any part of an organism, from the smallest
organ to the whole body, is liable to be repeated, with or
without differences between the repeated parts. This
phenomenon has been called merism : we have seen it
in cilia, trichocysts, contractile vacuoles, cells, limbs,
zooids, etc. Sometimes, as in cilia, cells, or zooids of the
same kind, it has not involved differences. Sometimes,
as in cells or limbs or zooids of different kinds, it has
involved differences between the repeated parts. Some-
times-'the parts are present in their full number from the
first ; sometimes they increase in number as growth goes
on. From time to time every organism produces a part
which not only repeats its whole structure, but also
separates from it by an act of fission. This process is
reproduction. In some cases of reproduction, as in the
budding of Hydra , the repetition of structure takes place
before separation. In other cases, as in the formation of
germs,1 the part which separates is simple in structure,
but has the power of repeating the structure of the parent
body after fission.

The term individual , whose application was in question
in the foregoing paragraph, has been used in
individuality. z00}0gy with very different meanings, which

are well illustrated by the life-history of the hydroids.

1 In this case there may be the additional complication that two such
separated parts so develop as to produce but one body after fusion.

MANUAL OF ELEMENTARY ZOOLOGY

An individual is a single complete living being. There
are in Obelia three things that might claim to be this :
(i) Since the medusa carries out all the functions of life
m itself, it seems natural to assert that it is a complete
living being, and since, as we have seen, its structure is
essentially that of a polyp, we might assume that each
polyp is also an individual. (2) On the other hand, the
whole polyp stock is a unit, and we might consider it to
be one individual, of which the separate polyps are
members, still regarding the medusa as an individual.
(3) From this we might go further, and claim that, as
the medusa is morphologically equivalent to one polyp
head, it is but a member of the individual to which the
polyps belong, though for purposes of distribution it
separates trom them, and thus we might regard as one dis-
joined individual the whole mass of forms which arise from
the fertilised ovum which gave rise to the polvp stock.

Of these alternatives the first is open to the objection
that it ignores the fact that the stock equally with each
ol its polyps may be regarded as a whole since it has
common nourishment and reproductive organs (the
gonangia), and obliges us to regard as individuals the
blastostyles, which are morphologically equivalent only
to parts ol other individuals. The third alternative does
indeed recognise what is a true entity, since the develop-
ment of every ovum (including those which develop
without fertilisation) does make a new start in that it
creates organisation anew, and with variation owing to a
shuffling of the contents of the chromosomes (p. 138) ;
but to regard such entities as individuals would compel
us to hold, not only that all the vast host ot independent
living beings which arise by the asexual reproduction of a
protozoon are parts of one individual, but also that the
“ identical ” twins formed by the dividing of a single
ovum are not, even in the case of man, separate individuals,
and this amounts to a reductio ad absurdum . The re-
maining (second) alternative is that which we have adopted
above. It may be stated as follows : u Every continuous
mass of living matter which arises normally by fission
is an individual.” That view has the advantage that it
does not force us to create artificial units of any kind.

C CEL ENTER A TA

233

According to it, the act which makes an individual is
the act of fission by which it becomes independent of its
parent, and fertilisation is the blending of two undeveloped
individuals into one, while the polyp stock is an individual
which contains a number of units meristically repeated,
and the medusa is an individual consisting of one unit of
the same type as those which exist in the polyp stock.

The essence of organic unity — that which causes a mass
of living matter, whether zooid or free “ indi-
f£ctoUrmfy,ng vidual,” so to be organised and to behave that
the activity of each part is subordinated to that
ot the whole — is as yet ill understood. But it would seem
to be connected with a dominating influence exercised by
certain parts which become the seat of activity, and there-
fore perhaps of a higher rate of metabolism, over the other
parts. Thus in the development of a hydroid colony the
apical region is the seat of an activity which brings about
the formation of a hydranth there, while the basal part,
dominated by the apex, remains less active, and merely
grows in length, until by this process some point upon
it becomes sufficiently removed from the apex to allow
the origination of a new hydranth, which in turn dominates
its own branch. So the body of the colony is shaped,
each new member coming into existence when and where
a previous dominating member permits. After this
fashion, it is suggested, though with infinite complica-
tions and the intervention of many other factors, the
bodies of all individuals are built up. The continuous
fall in metabolic activity between two points, which on
one theory is the cause of these phenomena, is known
as a metabolic gradient.

Other kinds ot polyps and medusae are known than those

Goeienterata w^ich are represented by Hydra and Obelia.

Sea-anemones ( Actiniaria : Fig. 532) are polyps
of rather complex structure, the edge of the mouth being
tucked in to form a gullet lined with ectoderm, while folds
or mesenteries of endoderm stretch across the enteron from
the body-wall, some ot them {primary mesenteries) reach-
ing the gullet. The testes and ovaries are developed from
the endoderm of the mesenteries, and their products are
shed through the mouth. The ectoderm is ciliated, and the

234

I

MANUAL OF ELEMENTARY ZOOLOGY

mesoglcea contains cells ol various kinds derived from the
ectoderm and endoderm.1 At the ends of the oblong gullet
are two grooves, the siphonoglyphs , down one of which and
up the other currents of water flow, even when the rest of
the mouth is closed. As in Hydra , the tentacles are hollow
and there is no medusa, though a planula larva is found.
Corals are related to anemones, but possess a calcareous
skeleton which sometimes, as in the Red Coral, lies in the

Fig. 158— A diagram of a vertical
section of a sea-anemone.

ecC, ectoderm of tentacle ; ect.g., ectoderm
lining gullet ; end., endoderm ; ent .,
enteron ; g., gullet ; gon., gonad ; m.f ,
mesenterial filament ; mus., longitudinal
or “retractor” muscle; w us'., oblique
or parietal ’muscle; st.l., mesoglcea ;
ten., tentacle; i°mes. , primary mesen-
tery.

jelly, but in the reel corals or madrepores is secreted on
the outside of the ectoderm of the lower part of the body.
Thus the “ coral insect ;; is a polyp. The large jelly-fish
(- Acalephce ) differ from the little medusae of the hydroids
in having no velum or nerve ring, and developing their

1 Though these cells do not form a third layer or “ mesoderm ” such
as is found in the higher animals (p. 275), yet they foreshadow that
dispersed element of the mesoderm which is known during develop-
ment as “ mesenchyme ” (p. 62 q).

A

B

Fig. 159. — Diagrams of transverse sections of a
sea-anemone.

A., Through the gullet ; B, below the gullet.

d.mes., “ directive ” mesentery ; ect., ectoderm ; end., en~
doderm ; ent., enteron ; m.f., mesenterial filament ; mus.,
muscle ; spg., siphonoglyphs ; st.l., mesoglea ; 1 °mes,,
2 °mes., 3 °mes., primary, secondary, and tertiary
mesenteries.

236

MANUAL OF ELEMENTARY ZOOLOGY

generative cells on the endoderm of the stomach v ail ;
their radial canals are often branched. Under the four
generative organs lie subgenital pits of the subumbrella
ectoderm. When there is a polyp generation ( hydra-tuba ),
it has solid tentacles and four mesenteries, and gives rise to
the medusa, not by budding, but by dividing transversely

Fig. 160. — A small specimen of the Common Jelly-fish
Aurelia aurita, natural size.

Note the horseshoe-shaped gonads, showing through the transparent tissues; the
radial canals, alternately branched and unbranched ; the little sense tentacles in
notches each opposite the middle branch of a canal; the marginal tentacles;
and the arms of the manubrium, each folded and fringed.

Water circulates from the stomach by the unbranched (adradial) canals to the
ciicular canal, and back by the branched (per- and inter-radial) canals.

into slices ( strobilation ), so that it appears for a while like
a pile of saucers. Each saucer then floats off, turns over,
and becomes a little jelly-fish or ephyra , which grows and
takes on the adult form. It seems likely that this curious
process arose by a polyp acquiring the habit of breaking
off its free end and sending it floating with the gonads to
distribute the species, and that then this came to be

Fig. 161. — A diagram of a vertical section through one of the large
jellv-fishes, such as Aurelia. The section is divided by a dotted
line into two halves, in one of which it is supposed to pass through
a radial canal, and in the other through the endoderm lamella.

can.c., Circular canal ; can.r., radial canal ; en.L, endoderm lamella ; ex.u., ex-
umbrella ectoderm ; g., gonad ; g.f., gastral filament ; lid., hood covering
sense tentacle ; j., jelly or mesoglea ; mb., manubrium ; ces., oesophagus
s.g.p., subgenital pit ; s.ten., sense tentacle ; st., stomach ; ten. tentacle.

Fig. 162. — The life-history of Aurelia.

A, Planula ; B-H, stages in the development of the hydra tuba ; /, ephyra.

237

238

MANUAL OF ELEMENTARY ZOOLOGY

repeated before the first slice was set free. In any case
w fie the medusa of a hydroid probably represents a whole
polyp developed tor a floating life, that of the hydra-tuba
is only the top of a polyp remodelled, though the structure
o polyps is such that the result in the two cases is substanti-
ally the same. All these and other animals whose structure
is fundamentally that of a polyp or medusa are known as
Lcelenterata.

We have seen that Hydra, like the Protozoa, is liable to

Beat* m the 'he ,Slcknes;! known as “ depression,” and this
Ccsienterata. may prove fatal. But there is no evidence that
Hydra , any more than the Protozoa, is subject
to that natural death which awaits the frog. The same
thing may be said of many other coelenterate polyps
though the Medusa, exhausting itself in producing the
reproductive cells, comes to die. This enduring life is
probably due to most of the cells which compose the body
of a . polyp being less highly differentiated for special
functions than those of the tissues of a frog, and is a very
interesting and important fact, since it shows us that the
mortality of the body cells (p. 150) :s not an unfailing
difference between them and the germ cells, but is a con-
sequence of the specialisation of most of them. What may
be the cause of the connection between specialisation and
mortality it is not easy to say. Possibly in the devotion of
cells to the performance of a special function the faculty of
excretion is impaired, so that eventually they are poisoned
by their own waste products. More probably the secret is
that the course of metabolism brings about in protoplasm
some change which hampers its working, and that specialis-
ation abolishes the power of renewing the parts which have
undergone this change. It would seem from the wasting
of the tissues ot old animals that, in a specialised tissue

wuu StmCtures . can be destroyed but not rebuilt!
Whether, again, it is the nucleus, or the cytoplasm, or
both, that are m fault does not appear, though the case
ot Faramectum , which has from time to time to destroy
its meganucleus (p. 169), suggests that the nucleus is at
least partly responsible. In any case it is interesting to
notice that the highly differentiated Ciliata show the same
weakness which attends differentiation in the Metazoa.

CHAPTER XIII

FLATWORMS

Divers of the lower animals are popularly known as
" worms.” They have little in common save
that their bodies are longer than they are
broad and have bilateral symmetry, and that their organisa-
tion is rather simple. The lowliest of such creatures are
known as the Flatworms or Platyhelminthes . As their
name implies the bodies of these worms are flat. They
have no anus and no blood vessels or body cavity, being
constructed internally of a spongy mass of tissue

ey*

Fig. 163. — A turbellarian {PI an aria polychroa). — From
Shipley and MacBride.

c.sl., Ciliated sensory slit at side of head ; eye ; g.o., genital opening ; m., mouth, at
end of outstretched pharynx ; pli.s., sheath into which pharynx can be withdrawn.

(parenchyma, p. 243), containing muscle fibres, and,
imbedded in this, a gut (except in the tapeworms), a
nervous system with a rudimentary brain, an excretory
system formed of branched tubes ending internally in
ciliated “flame cells” (p. 243), and a complicated, nearly
always hermaphrodite generative system.

Some of the flatworms lead a free, if unobtrusive,
existence in fresh waters or the sea or in damp
places. They, are known as Turbellaria . The
epidermis on the surface of their bodies is a columnar
ciliated epithelium which contains gland cells and sense

239

Turbellaria.

24o MANUAL OF ELEMENTARY ZOOLOGY

cells ol \ arious kinds. 1 he sense cells are most numerous
at the front end, which constitutes a rudimentary head,
and they may be raised on tentacles or sunk in pits ;
there are usually also eyes. The worms crtwl on the
bottom 01 under the surface film ol the waters by means
ol their cilia, and the larger kinds swim by muscular
undulations ol the body. 1 hey are carnivorous and

predatory, seizing their prey by means of a
muscular pharynx, which can be protruded
as a funnel. These little creatures are not
of great importance either to man or in the
economy of nature, but one leature of their
organisation calls lor our attention. In
them better than in the more important
parasitic flatworms there can be studied
the simplest kind of brain. It consists of a
pair of ganglia in the head, united by a
commissure and giving off longitudinal
nerves to the body. These supply a nerve
net under the epidermis and another deeper
in the body. The function of this brain is
merely to relay and distribute the impulses
from the important sense organs on the
head. Unlike Hydra , where no part of
the body permanently dominates the rest,
these creatures, moving as they do always
with the same end forwards, have that end
organised for perception and the con-
sequent stimulation of the rest of the
body. This permanent organisation of a
dominant region of the body unifies the
reaction ol the body as a whole to changes
in its surroundings. It brings into being
a brain and nerves, and these have further advantages
(see p. 215). But in the Turbellaria the brain does not
co-ordinate the activities of different regions of the body.
It sets them in action: that each plays its proper part
is due to local organisation.

Much more important to man than the Turbellaria are
two classes of flatworms that are parasitic — the Flukes
or Trematoda and the Tapeworms or Cestoda. We have

diagram of the
Nervous System
of a Turbellarian.

c.g. brain ; e., eye ;
l.n., longitudinal
nerve cord ; o.,
opening through
which the
pharynx is pro-
truded; ph.,
pharynx ; ph.s.,
pharynx-sheath.

FLATWORMS

241

now to proceed to the study of some examples of these

Trematoda groups. Sheep which are fed in damp

the Liver meadows are liable to a serious and usually

Fluke- fatal disease known as “ liver rot,” in which the

wool falls off, dropsical swellings appear, and the animal
wastes away. This has been found to be caused by
a parasite known as the Liver Fluke, Distomum hepaticum
or Fasciola hepaiica , which lives in the bile ducts
of the sheep and sometimes of other animals, in-
cluding occasionally man. It is a flat, brownish worm,
about one inch long by half an inch broad, shaped like a
leaf with a blunt triangular projection
at the broader end. At the tip of this
projection lies the mouth, in the
midst of an anterior sucker , and just
behind the projection an imperforate
posterior or ventral sucker is placed
in the middle of the ventral side and
serves as a means of attachment.

Nearly midway between the suckers
is a smaller genital opening , at the
hind end of the body is a minute
excretory pore , and on the dorsal
surface at about a third of the length
of the animal from the front end lies .

T c . J r.o., Genital opening; m.,

the opening of the Faurer-Ftzeaa mouth; v.s.s ventral)

canal presently to be mentioned. sucker.

The bodv is covered with a cuticle,

in which little backward-pointing spinul.es are embedded.

The mouth leads into an ovoid, muscular pharynx, from
which a short oesophagus passes backwards to
Gut- divide before the posterior sucker into right

and left branches or intestines, which run on either side ol
the middle line to the hind end of the body, giving off on
either side many offsets, which in turn are much branched.
There is no anus. The worm feeds on the juices, probably
normally on the blood, of its host.

The ectoderm cells have sunk inward after secreting the
Layers ot cuticle. Below the cuticle lie successively

the Body. circular, longitudinal, and diagonal layers ot

muscle fibres, with the epidermal cells among the
16

Fig. 165. — The Liver
1 mke.

MANUAL OF ELEMENTARY ZOOLOGY

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FLATIVORMS

243

longitudinal fibres. Between these and the endoderm,
which is a columnar epithelium lining the gut, lies the
parenchyma , a, mesh-
work of protoplasm with
nuclei at the nodes and
oval cells in the meshes.

Muscle fibres pass across
the parenchyma from
the dorsal to the ventral
side of the body. There
are no blood vessels or
coelom. It will be
noticed that in the fluke
a mass of tissue lies
between the ectoderm
and endoderm in place
of the structureless lam-
ella of Hydra . This is
known as the mesoderm.

We shall allude to it in
more detail in describing
the earthworm.

The excretory system
lies in the

System017 parenchyma.

It consists of
a meshwork of tubules
joining into a main duct
which lies in the middle
line, from a point about

a quarter of the length fIG- 167. — The structure of a liver fluke.

— AfterSommer. From the ventral
surface. The branched gut ( g. ) and
the lateral nerve (/.«.) are shown to
the left of the figure, the branches
of the excretory vessel (e.v. ) to the
right.

c.s., Position of cirrus sac ; eg., lateral head
ganglion ; m., mouth ; ph., pharynx ; v.s.,
ventral sucker. An arrow indicates the
excretory aperture.

of the body behind its
front end to the ex-
cretory pore at the hind
end. The ultimate
branches of the tubules
are very fine and end
in little structures known
as flame cells. These
are minute vesicles containing a few long cilia which keep
up a flickering like that of a flame and so perhaps drive

244

MANUAL OF ELEMENTARY ZOOLOGY

towards the main duct the fluid secreted into the vesicle
by its walls. Each vesicle has a nucleus and may be regarded
as a hollow cell. It is connected with its fellows by fine
protoplasmic processes which are said to be hollow.

The nervous system includes a brain which consists of
a collar around the pharynx with a mid-ventral
swelling and a pair of lateral swellings. From
these swellings or ganglia nerves are given off
to the forepart of the body, and from each lateral ganglion

Nervous

System.

Fig. i 68. —Two flame cells, highly magnified.

arises a large lateral nerve cord which runs backwards
below the gut to the hinder end of the body, giving off
branches on the way. The nerve cords contain nerve cells
as well as fibres.

The liver fluke is hermaphrodite, and has very complex

Generative generative organs.

Organs.

I he testes are two much-branched tubes lying one
behind the other in the middle part of the body. The branches of each
are gathered into a vas deferens, and the two vasa deferentia run forwards,
sidebysidetojoin, abovetheposteriorsucker, alarge, pear-shaped vesicula

FLA TWO XJlfS

245

seminalis. From this a tine, somewhat twisted tube, the ductus ejacula-
torius, passes forwards to enter a stout, muscular penis or cirrus , which
opens at the generative pore. Normally the penis lies in a cirrus sac ,
but it can be turned inside out and thus thrust out of the pore. The
ovary is a branched tubular structure on the right side in front of the
testes. Its branches join to form the oviduct, which passes towards
the middle line and there joins the yolk duct. This is formed by the
union of two transverse ducts, which lead each from a longitudinal
duct at the side ot the body. The yolk glands are very numerous, small,
round vesicles lying along the sides of the body, and communicating
by short ducts with the longitudinal ducts. The Laurer-Stieda canal
is a short tube of uncertain function leading from the union of oviduct
and yolk duct to a pore on the back. Possibly it is used for the re-
ception of spermatozoa from another individual. The oviduct and
yolk duct are surrounded where they join by a rounded mass, which is
known as the shell gland though its function is probably only to
harden the egg shell, which appears to be secreted by the yolk glands,
composed of numerous, minute, unicellular glands. From this point
the joined ducts proceed forwards as a wide, twisted tube, the uterus,
to the generative opening. The uterus contains eggs, each enclosed
in a shell, within which lie, besides the ovum, a number of yolk cells
derived from the yolk glands, and spermatozoa. The animal is
probably as a rule self-fertilised.

Life- History.

The life-history of the liver fluke is a very remarkable
and interesting process. The eggs, which are
very numerous, are laid into the bile ducts of
the sheep. So long as they remain within the body of
the latter they do not develop, but when they have been
carried by the bile to the intestine, and thence passed
to the exterior with the droppings, they will develop in
damp spots if the weather be warm. In a few weeks a larva
known as the miracidium emerges. It is conical, covered
with a layer formed by five rings of big, ciliated cells,
provided with two eye-spots, a small gut, and two flame
cells, and filled by a mass of cells. It swims by means of
the cilia, with the broad end forwards. At this end there
is a knob which can be thrust out as a conical spike. If it
can find a member of a particular species of water snail
known as Limnceus truncatulus 1 it works its way into the
tissues of the snail, thrusting out its spike and rotating by
means of its cilia so as to bore in. Within the snail the

1 Other species ot water snail are sometimes used in foreign
countries,

246

MANUAL OF ELEMENTARY Z CTO LOGY

y-N

h.g
v.d

ciliated cells are lost and the larva increases in size and
grows into a hollow sac or sporocyst. Sometimes this

s.v

T

Fig. 169. — The reproductive organs of a liver fluke, from the
ventral side. — After Sommer.

c.s. Cirrus sac.

/. Female aperture.
g. Anterior lobes of gut.
m. Mouth.
ov. Ovary (dark).
p. Penis.

s.v. Seminal vesicle.
sh.g. Shell gland.

T. Testes (anterior).
ut. Uterus.
v.d. Vas deferens.
y.gl. Diffuse yolk glands.

multiplies by dividing transversely. Within the sporocyst
some of the cells lining the cavity behave like fertilised ova,

FLATWORMS

247

Fig. 170* — The life-history of a liver fluke. — After Thomas.

1. Developing embryo in egg-case ; 2. free-swimming ciliated embryo ;
3. sporocyst ; 3 a. shell of Limmeus truncatulus ; 4. division of
sporocyst : 5. sporocyst with redite forming within it ; 6. redia with
more reuiai forming within it ; 7. tailed cercaria ; 8. young fluke

248

MANUAL OF ELEMENTARY ZOOLOGY

dividing to form a blastula, which invaginates to give rise
to a twodayered sac or gastrula. These cells, however,
have not undergone fertilisation, and their development is
an example ot parthenogenesis , the development of un-
fertilised ova. The gastrula grows into another form
of larva, the redia , which bursts out of the sporocyst
and migrates, usually into the liver of the snail. The redim
devour the tissues of the snail and finally kill it. Each
redia has an elongated body with an anterior mouth, a
muscular pharynx, and a short, sac-like gut. A little way
behind the pharynx the body-wall is thickened to form a
muscular collar, and not far from the hind end are two
blunt conical processes on one side. Posteriorly there is a
large body cavity lined by an epithelium like that of the
cavity in the sporocyst. Cells derived from the wall of the
body cavity develop in much the same way as in the sporo-
cyst and give rise to daughter rediae, which escape from the
parent by an opening behind the collar. Several generations
of redim usually succeed one another thus, but eventually
they cease to produce daughters of their own kind, and give
birth instead to creatures known as cercarice , with a flat,
heart-shaped body, two suckers, a forked gut, and a tail.
The cercaria emerges from the redia, works its way out of
the snail, and swims by means of the tail. Soon it settles
upon a wet blade ot grass, loses its tail, secretes around
itself a cyst by means ot special cystogenous cells of the
ectoderm, and waits till the grass is eaten by a sheep. In
the gut of the latter the cyst is digested and the cercaria
pierces the intestinal wall, bores into the liver, and there
grows into an adult duke. When the generative organs are
fully developed, the worms begin to lay their eggs, and
migrate to the duodenum of the host. In the end they are
cast out with the faeces, and if the sheep survives till this
happens it will usually recover, though, owing to permanent
damage to the liver, the recovery is never complete.

It will be seen that in this life-history we have a case of
alternation of generations far more complicated than that
ol Obelia , and differing from the latter also in that not
sexual and truly asexual, but sexual and parthogenetic
generations succeed one another. The former kind of
alternation of generations is known as ??i et agenesis , the

FLA Tl FORMS

2 49

latter as heterogamy . It should also be noticed that there
are three kinds of individual involved in the cycle. The

I s- generation

2 ’'-“generarion
S'? generation

Egg — M i racidium s-Sporocysr

I

Sporocysf

Redia

1

Cercaria — >Aduir Redia

Schistosoma.

Fig. i 71. — A diagram of the life-history of the liver fluke.

life-history of the liver fluke is shown by a diagram in
Fig. 171.

Platyhelminthes which, like
the liver fluke, are
parasitic and covered
with a cuticle, and possess
suckers and a gut are known as
Trematoda . As another example
of them, we may notice Schisto-
soma (or Bilharzia ), which lives
in the veins of man and is excep-
tional in having separate sexes
and remarkable in that the
female is carried in a groove of
the ventral surface of the male.

Eggs are laid in the walls of the
intestine and bladder, causing
inflammation, and thus reach
the exterior. The intermediate
host is a snail, from which the
cercariae pass into water. In-
fection takes place by the cer-
cariae penetrating the skin or
the mucous membrane of the
mouth of the final host.

To the Platyhelminthes be-
long also the Tape-

w or ms or Cestoda , of which Tcenia solium , found
in man, is an example. In the adult state this worm may
reach a length of nine feet. It lives in the intestine, to

Fig. 172. — Sck istoso »i a
hcematobiiun. — From
Sedgwick.

male ; 9 , female ; X, sucker.

Cestoda.

250

MANUAL OF ELEMENTAL Y ZOOLOGY

whose wall it is attached by a head or scolex , provided with
four suckers and a crown of hooks. Behind the head is a
narrow neck, followed by a long chain of joints or proglot-

Fig. i 73- The life-history of Tenia solium. — After

Leuckart.

t. Six-hooked embryo in egg-case ; 2. proscolex or bladder-worm
stage, with invaginated head : 3. bladder-worm with evaginated
head ; 4. enlarged head of adult, showing suckers and hooks;
5. general view of the tapeworm, from small head and thin
neck to the ripe joints ; 6. a ripe joint or proglottis with branched
uterus ; all other organs are now lost.

FLATWORMS

251

tides which it buds off. The younger of these, near the
head, are small, but they grow larger as they are pushed
farther from the head by the formation of new joints. The
body is covered with a cuticle, under which lies a layer of
circular muscle fibres and then one of very deep epidermal
cells with longitudinal muscle fibres between them and a
transverse layer of muscle below them. Inside this is a
mass of parenchyma like that of the fluke, in which are

with the reproductive organs at the stage
of complete development.

c.s., Cirrus sac ; excr., excretory canals ; g.o., genital
opening ; n.c., nerve cord ; ov ovary ; s/t.g.t
shell gland; t. , testes; v.d., vas deferens; ut.,
uterus ; vag., vagina; y.g., yolk gland.

embedded the excretory, generative, and nervous systems.
There is no alimentary canal, nutriment being absorbed
through the surface of the body. The excretory system is
of the same type as that of the fluke, with flame cells and a
larger and a smaller main duct on each side, connected by
a transverse vessel on the hinder side of each proglottis.
In the last proglottis these vessels open by a median pore.
The nervous system consists of a ring in the head, small
forward nerves, two lateral nerve cords and branches. The

252 MANUAL OF ELEMENTAL Y ZOOLOGY

reproductive organs have the same general structure as in
the liver fluke : they are shown in Figs. 174 (and 173, 6).
Each proglottis contains a complete set of them. It is
fertilised either by another proglottis or by itself. From
time to time the last proglottis breaks off, singly or with
others, and passes out of the anus. It has some power of

Fig- 175- — A transverse section through a proglottis of Teem a in
which the reproductive organs are well developed.— From
Shipley and MacBride.

.I, Cuticle , 2 long-necked cells ot ectoderm ; 3, longitudinal muscle fibres cut
across , 4, layer of transverse muscles ; 5, split in the parenchyma which lodges a
calcareous corpuscle ; 6. ovary ; 7, testis with masses of male germ-mother-cells
forming spermatozoa ; 8, longitudinal excretory canal ; 9, longitudinal nerve
cord ; 10, uterus ; 11, oviduct.

independent movement by contraction of its muscles. The
<?ggs are set free by its rupture. If, as may happen in vari-
ous circumstances, they are now swallowed by a pig, or, as
occasionally, by man, their shells are dissolved in the ali-
mentary canal and a little spherical six-hooked embryo or
■onchosphere is set free from each. This bores its way from
the intestine ol the host into his blood vessels and is carried
to the muscles and other organs, where it loses its hooks, in-
creases in size, and becomes a bladder-worm or cysticercus.
d he wall ol this becomes tucked in at one spot, forming a
pouch, on the inner wall of which the suckers and hooks of
the future head appear. No further change takes place
unless the flesh of a pig infested with such bladder- worms
(known as measly pork) be eaten raw or u underdone ”
by man. When this happens, the stimulus of the new sur-
roundings causes the head to be turned inside out. The

FLATWORMS

2 5 >

bladder is digested, but the head fixes itself and begins to
bud off proglottides. The life-history of T tenia solium
may be summed up as follows : —

Egg->Onchosphere->Cysticercus->Scolex^ Adult.

It will be seen that only one generation is involved,.

Fig. 176. — Diagrams of bladder-worms. — From Thomson.

I. The ordinary Cysticercus type, with one head (H.).

II. The Coenurus type, with many heads.

III. The Echinococcus type, with many heads, and with
secondary cysts or brood capsules ( B.C .) pro-
ducing many heads.

unless each proglottis be regarded as a complete individual,
and not merely as a part of the parent body broken oft to*
carry the eggs.

Other common tapeworms are : Tcenia saginata , without
other hooks, found in man, with the bladder-worm

Tapeworms. stage in the ox ; T. serrata in the dog, with a
bladder-worm in rabbits, hares, and mice ; 1 . ccenurus'

254 MANUAL OF ELEMENTARY ZOOLOGY

in the dog, with the bladder-worm known as Ccenurus
cerebrahs in the brain of sheep and other hoofed
ammals, where it causes “ staggers ’ ; and T. echinococcus,
which has only three proglottides, in the dog, with the
bladder-worm Echinococcus in sheep, oxen,' pigs and
sometimes m man. The latter two species produce in the
bladder-worm stage numerous heads. Since only one of
these can be regarded as continuing the individuality of the
bladder-worm, the others must be looked upon as buds
Irom it, so that there is here a metagenesis. The bladder
produced by T. echinococcus , is known as a “ hydatid
cyst ; it is very large, containing sometimes as much as

a gallon of fluid, and its wall buds off secondary cysts into
the cavity.

The harm which tapeworms do by robbing their host of
lood is generally insignificant. Their action in setting up
nntation in the intestine and in secreting substances which
prove poisonous, especially to the nervous system is more
serious. They may be avoided by care taken in regard to
tood, and are treated with vermifuges, such as extract of
male fern, and purgatives. The cysticercus stage is be-
yond the reach of medical remedies, but may sometimes
be treated surgically.

CHAPTER XIV

THE EARTHWORM. ANNELIDA. TRIPLG-
BLASTIC ARCHITECTURE

Almost everywhere in England earthworms are found.

They live usually in the upper layers of the soil
in burrows, which they make partly by boring
with the pointed front ends of their bodies, partly by
swallowing the earth in front and passing it out behind, in
which case the earth which is passed out forms the well-
known “worm casts.” The sides of the burrow are lined
with a slime secreted by unicellular glands in the skin, and
if the opening be not protected by a worm cast it is usually

Fig. 177- — The Earthworm. — From Thomson.

closed by leaves or small stones. Such leaves may often be
seen sticking up from the ground, and will be found to
have been pulled into the burrow skilfully, with the
narrowest part foremost. At night, if the weather be warm
and not too dry, the worms will stretch themselves out of
their holes, keeping the hinder end of the body fixed in the
opening, so that they can pull themselves back at once if
danger threatens. In dry weather or hard frost they burrow
deep and retire to a small chamber, which they line with

little stones. In wet weather they are sometimes flooded

255

256

MANUAL OF ELEMENTARY ZOOLOGY

out, but they rarely leave their burrows in other circum-
stances, except when they are about to die owing to the
attacks of parasitic maggots which are the young of certain
flies. The food of earthworms consists of the organic
matter in the soil, which they swallow, and of leaves both
fresh and decaying. They will also eat animal matter, and
are said to be very fond of fat. Charles Darwin has shown
the remarkable effects which these insignificant creatures
have upon the surface of the earth. By making the soil
more porous they expose the underlying rocks to the
disintegrating action of water, by solution owing to the
presence of carbon dioxide and other acids of the soil, and
by frost; and the small stones which eventually result from
this action are made still smaller by friction and solution
within their bodies. Thus they help in the formation of
the soil. At the same time they are aiding in its removal.
Their castings dry and crumble, and are blown about by
the wind or else are washed down by the rain. On sloping
ground this fine material tends to be carried away down-
wards, and thus the denudation of hills is largely due to
the action of earthworms. On the other hand, their work
is highly beneficial to the farmer. The soil is by them
thoroughly mixed, submitted to the action of the air, and
constantly supplied with a fine “top dressing.” It has
been calculated that earthworms bring up annually a layer
of soil one-fifth of an inch in thickness, which is spread by
the weather in the way we have described. Organic matter
is converted into a useful form and amalgamated with the
earth, and the latter is made easier of penetration by the
roots of plants.

The commonest English earthworm is Lumbricus herculeus.

The body of this animal is roughly cylindrical.
Features. but pointed in front and broadened behind. It
reaches a length of seven inches. There is no
distinct head, but a lobe known as the prostomium over-
hangs the mouth, which is a crescentic opening on the
lower side of the front end. The body is divided into a
series of rings, the segments or somites , and at the hinder
end is the terminal anus. The first somite is the peri-
stomium and the mouth lies between it and the prostomium.
On the dorsal side, the latter projects across the peri-

THE EARTHWORM

257

m

stomium.1 There are about 150 somites. At about one-
third of the length of the body from its front end, in

somites 32-37 inclusive,
a glandular thickening of
the epidermis lies ath-
wart the back like a
saddle and is often mis-
taken for the scar of a
wound. This is known
as the clitellum . The
skin of the worm is
brownish above and
paler below ; it is covered
with a fine, tough, iride-
scent cuticle secreted by
the underlying cells. In
every somite except the
first and the last there
are eight bristles, the
chcetce , in two pairs on
each side, a lateral pair,
slightly above the
middle of the side, and
a ventral pair between
the lateral and the
mid-ventral line. The
chaetae can be felt with
the fingers ; they consist
of a horny nitrogenous
organicsubstanceknown
as chitin and are em-
bedded in sacs of the
epidermis, by whiph they
are secreted, and to these
sacs are a ttached muscles,
by which they can be
moved. The chaetae, as
we shall see later, are
organs of locomotion. The ventral chaetae of the clitellum,

1 In the related Allolobophora the prostomium reaches only half-
way across the peristomium.

17

bricus hercn/eus ), from the
right side ; B, the first four
segments from below; C , the
same from above ; D (after
Grove), worms in coition.

elm., Clitellum ; l.ch ft., lateral chaetae; m.
mouth: per-, peristomium; pro.

prostomium ; 5.2 s. 32, 9 37 numbers
of segments; sec., clitellar secretion
uniting worms; sp.^r., spermatic
groove ; v.l h t. ventral chaetae ; ^ ,

opening of vas deferens ; 9 , opening
of oviduct.

258 MANUAL OF ELEMENTARY ZOOLOGY

er.t.

External

Openings.

of the twenty-sixth, and of the tenth to the fifteenth somites

are straighter and more
slender than those of
other somites, which are
stout and somewhat
hooked. This modifica-
tion is in connection
with the use of the
chsetse of the twenty-
sixth somite during
coition, and of the other
straight chaetae during
the formation of the
cocoon in which the
eggs are laid.

A number of internal
organs open
separately
upon the
surface of the body.
We have already men-
tioned the mouth and
anus. The openings of
the vasa defere?itia are
a pair of slits with
swollen lips found on
the under side of the
body in somite 15. In
front of them, in somite
14, are the two small
openings of the oviducts . The spermathecal pores are two
pairs of small, round openings in the grooves between
■somites 9-10 and 10-11 at
the level of the lateral
•chaetae. The nephridiopores
are openings which lead
from the excretory tubes or
nephridia. They are found,
as a pair of minute pores
in front of the ventral
chaetae, in each somite except the first three and the last.

Fig. 179. — A diagram of a chaeta of
the earthworm and the structures
connected with it. — From Potts,
after Stephenson.

c.m., Circular muscle of body-wall ; ch.,
chaeta ; cu., cuticle ; ect., ectoderm ;
fol., follicle, and fm.c., formative cell of
chaeta ; per., peritoneum ; pr.m., pro-
tractor, and rt.m., retractor muscles of
chaeta.

Fig. i So.- — One of the ordinary
chaetae of an earthworm,
removed from the chaetae
sac and magnified.

THE EARTHWORM

259

The dorsal pores are small, round openings on the mid-
dorsal line in the grooves between the somites. The first
is behind the eighth somite, and there is one in each
subsequent groove. They open into the body cavity, the

d. b. v.

Fig i 8 1.— A tranverse section through an earthworm in the region

of the intestine.

cce.. Coelom ; c.c., chloragogenous cells ; c.m.b., circular muscle of body-wall ; c.m.g.,
circular muscle of gut ; c.s., chseta sac ; c.s.m., chaeta sac muscles ; cm., cuticle ;
d.b.v., dorsal blood vessel ; ep., epidermis , end., ednoderm ; g.f., giant fibres ;
l.m.b., longitudinal muscle of body-wall ; l.m.g., longitudinal muscle of gut ;
l.n.v., lateral neural vessel ; n., nerves ; n.c., nerve cord ; nph., nephridium ;
p.e.b., peritoneal epithelium of body-wall ; chce., chaeta ; s.i.v., subintestinal
blood vessel ; s.n.v., subneural blood vessel , ty., typhlosole.

Some of the structures seen in this section are shown more highlv magnified in

Fig. 186.

fluid in which oozes out through them and moistens the
surface of the body, mingling with the slime secreted by
the unicellular glands of the skin. As this fluid contains
amoeboid cells which attack bacteria and other small
parasites, it is a valuable defence to the worm against
such enemies, which are numerous in the soil.

26o

MANUAL OF ELEMENTARY ZOOLOGY

The body of the worm may be said to consist of two
Body-wan. tubes, one within the other. The inner tube is
the gut, the outer the body-wall. Between the
two lies the coelom or body cavity, divided into compart-
ments by a series of septa , which reach from the gut to the
body-wall, where they are attached opposite the grooves on
the surface of the body. The compartments communicate
by numerous openings in the septa. The coelom contains
a fluid, and in this float the leucocytes already mentioned,
by which small parasites are surrounded and destroyed,
both within and without the body. The body-wall is
covered by a cuticle. Under this lies the epidermis , an
epithelium consisting of columnar cells, many of which
are glandular or sensory, with small cells between their
bases. The cuticle is composed of hardened protein and
is perforated by a pore over each gland cell. The epi-
dermis of the clitellum consists of several layers of gland
cells. Below the epidermis is a circular layer of muscle ,
consisting of unstriped fibres running around the body, and
below this again lies a much thicker longitudinal layer of
muscle , composed of similar fibres running along the body
and placed in rows which stand at right angles to the
surface, supported by connective tissue. Within the
longitudinal muscle is the ccelomic epithelium, which is
here a layer of pavement cells lining the body cavity.

The earthworm has a well-developed central nervous
system which consists of (i) a pair of supra-
Systemf pharyngeal ganglia , rounded bodies lying
above the mouth, and sometimes known to-
gether as the brain, (2) two slender circumpharyngeal
commissures running from these round the pharynx, and
(3) a ventral nerve cord which starts from the commissures
between the third and fourth somites and runs the whole
length of the body in the coelom below the gut, swelling
into a ganglion in each somite. The first of these ganglia
is bilobed and is known as the subpharyngeal ganglion.
Nerves are given off to the prostomium from the supra-
pharyngeal ganglia, and to the first two somites from the
commissures, and the ventral cord gives off in each somite
three pairs of nerves which run upwards as girdles in the
body-wall, giving off branches as they go. The alimentary

THE EARTHWORM

261

canal receives nerves from the ci rcumpharyngeal com-
missures and fibres from plexuses in the septa. Though
the ventral cord appears to be single, it is really double,

and can be seen m trans-
verse sections to be rather
imperfectly divided into right
and left halves by connective
tissue. Transverse sections
also show that the middle
and upper part of the cord
consists of fine, chiefly longi-
tudinal, nerve fibres, and the
lower and outer parts contain
nerve cells. Above the mass
of fine fibres are three longi-
tudinal bundles o±' such
fibres, each bundle being en-
closed in a sheath and known
as a giant fibre. Nerve cells
are more numerous in, but
not confined to, the ganglia.
The nerves consist of affer-
ent fibres, which start from
sense cells in the epidermis

Fig. 182.-— A diagram of a longi-
tudinal section of an earth-
worm.

a.v.s. Anterior vesicula seminalis.
a.v.s' . Posterior lateral horn of the
same overhanging the
oesophagus.
cr . Crop.
giz. Gizzard.
int. Intestine.
tn. Mouth.
aes. (Esophagus.

/ v.s. Posterior vesicula seminalis.
p.v.s'. Horn of the same overhang-
ing the oesophagus.
per. Peristomium.
ph. Pharynx.

■ pr. Prostominm. 1

sup p‘i.g. Supraphkryngeal ganglion
ty. Typhlosole.
v.n.c. Ventral nerve cord.
i-23. Segments.

The blood vessels are omitted.

/

y

V

✓

i

262

MANUAL OF ELEMENTARY ZOOLOGY

and muscles (Fig. 185) and end as bunches in the central
nervous system, efferent fibres, which start from nerve cells
in the ganglia and end against muscle and other cells, and
also fibres which join nerve nets in the skin, muscles, and
septa.

sup ph

Mechanism of
Locomotion.

The muscular and nervous apparatus is used in the
following manner to bring about the movements of the
worm. In ordinary locomotion it works somite by somite.
Simultaneous contraction of the circular musculature of

a somite and relaxation
of its longi-
tudinal
muscula-
ture causes the somite
to extend. Its chsetse
are then thrust out and
the action of the mus-
culature is reversed,
so as to shorten the
somite. Since the
chaetae, which point
somewhat backwards,
hold firm in the earth
and prevent any draw-
ing back, the result is
to pull forwards the
somite behind. In this
the process is being
repeated, and so a wave
of forward movement

sep-

v-S- •

Fig. 1 S3. — A diagram of the forepart of
the nervous system of the earthworm.

c.ph.c., Circumpharyngeal commissure;

nerves; ph., pharynx cut through; sep.,
septa; subph.g., subpharyngeal ganglia;
sup.ph.g., suprapharyngeal ganglia; v.g.,
ganglia of ventral cord ; i 7, somites.

passes back along the
body. The passage of the wave is due partly to a
mechanical stimulation of nerves of the muscles of a
somite by the pull from in front. I his is shown by an
experiment in which a worm was cut into two and the
anterior and posterior halves joined together by threads
hooked into the tissues. When the forward movement of
the anterior hall pulled upon the hinder somites these
performed the usual contractions. But if the body-wall
and gut be severed and the nerve cord left as the only
connection between the halves of the worm the wave of

THE EARTHWORM

263

Fig. 184. — An earthworm ( L . herculeus), dissected from above.

.v.s'. Horns of the anterior vesicula seminalis ; cr., crop; d.b.v., dorsal blood
vessel ; giz., gizzard ; ht.y hearts ; ini., intestine ; m., mouth ; nph., nephridia ;
css., oesophagus ; oes.gl., oesophageal glands ; css.p., oesophageal pouch , p.v.s
horns of the posterior vesicula seminalis ; ph., pharynx ; sep., septa ; sp.,
spermathecae ; snp.ph.g., suprapharyngeal ganglia.

264 MANUAL OF ELEMENTARY ZOOLOGY

contraction still passes normally, and in fact, as might be
expected, the principal agent in co-ordinating the, action

“ reflex
along

of the musculature is
the nervous system.
The contraction of
circular or longi-
tudinal muscles in
a somite causes,
through a
arc ” (p. 96)
the ventral cord,
the contraction of
the corresponding
muscles in the somite
behind. ^Further, in
each somite the con-
traction of the cir-
cular musculature
sends, ' by afferent
and efferent fibres,
through the gan-
glion, impulses which
relax the longitu-
dinal musculature ;
and similarly the
longitudinal muscles
in contracting relax
the circular. Sudden
movements of the
whole body, such as
those by which the
worm withdraws from
the region in which
it receives a powerful
stimulus — -which
probably indicates
danger— are brought
about by a different
nervous mechanism.

1 he giant fibres, which give off branches to the muscles of
each somite, are a means of direct communication between

Fig. 185. — A diagram showing the mode
of ending of the sensory nerve fibres
in the epidermis of the earthworm
and the relation of this type to that
which is found in most of the
sensory fibres of the frog. See also
Fig. 61.

A , The arrangement found in the earthworm ;
J9, That of thmworm *N,et>eds ;C, thatofxajfish ;
D, that of a frog or man.
c.n.s., Ending of the neuron in the central nervous
system ; cp., ending in the epidermis.

THE EARTHWORM

265

distant ganglia. They conduct mo^e rapidly than the
other fibres of the' nervous system, and it is by their
agency, conveying impulses to a number of somites at
the same time, that large, sudden movements are carried
out when one point on the body is strongly stimulated.

An earthworm has no well-developed organs of sense,
Sense Organs. but certain °f the columnar cells of the epider-
mis are rod-shaped and prolonged at their
inner ends into fibres, which run in the nervous system
(Fig. 185). These are sense cells, and in the forepart of the
body some of them are collected into groups, which are
rudimentary sense organs. There are also sense cells
which contain a refractive body and are probably affected,
by light. Experiment shows that the worms are sensitive
to light and to vibrations of the grofind and can smell,
but gives no evidence of a sense of hearing.

The alimentary canal is straight. It begins with a short,
Gut wide, thin- walled mouth onbnccaTcavity ih the

first three somites, which leads to a muscular
region known as the pharynx. This lies in front of the
septum between the fifth and sixth somites, but pushes that
septum backwards as far as the seventh. Its dorsal wall is
thickened by the presence of a number of glands, whose
secretion, containing mucin and a ferment which digests
proteins, is poured over vegetable tissues while the animal
is feeding upon them. Numerous strands of muscle run
from it to the body-wall. Behind it lies the oesophagus, a
straight, narrow, thin-walled tube, which extends to the
fourteenth somite. In the eleventh somite it bears at the
sides a pair of oesophageal pouches , and in the twelfth two
pairs of oesophageal glands. These contain large cells
which secrete calcium carbonate and pass it through the
pouches into the oesophagus. In the fifteenth and sixteenth
somites the oesophagus expands into a large, thin-walled
crop, which in turn communicates behind with the gizzard ,
another swelling, with thick muscular walls and a horny
lining, in somites 17 and 18. From the gizzard to the
anus runs a wide, thin-walled tube known as the intestine.
The intestine is narrowed where it passes through the septa,
and its dorsal wall is infolded to form a longitudinal ridge
known as the typhlosole. The cut is lined with a layer of

266

MANUAL OF ELEMENTARY ZOOLOGY

columnar epithelium, outside which are thin longitudinal
and circular muscular layers, covered by the coelomic epi-

i x — — niv wu y .

b.v., Blood vessel ; gl.c., gland cell in the epidermis. Other letters as in Fie- 188
(A and A ) and Fig. 181. 6'

thelium, which here consists of the chloragoge nous cells ,
These cells, which also fill the typhlosole,Care large and
contain yellow granules of an excretory product. " They
fall off into the coelomic fluid, and there’ break up and set

THE EARTHWORM

267

free their granules, which are taken up by leucocytes. It
is said that they are by these conveyed to the exterior,
probably through the skin or dorsal pores, and also that
they are deposited as pigment in the tissues. There are
also amoeboid yellow cells which take up excreta in the
blood, pass into the gut, and are voided with the faeces.

Food is drawn into the mouth by a sucking action of the
muscular pharynx, passed along the oesophagus, stored in
the crop, ground up in the gizzard with the aid of small
stones which have been swallowed, and in the intestine
first digested by juices secreted from the epithelium, and
then absorbed, for which processes the surface is increased
by the presence of the tvphlosole. The contractions which
cause the passage of the food are alternately caused through
the nerves to the pharynx and inhibited through the
plexuses in the septa. The function of the oesophageal
glands is probably the excreting of the calcareous matter
which is very plentiful in the dead leaves of which the
food is largely composed. Possibly their secretion is also
of importance in removing carbon dioxide in the form of
calcium carbonate.

Besides the chloragogenous and yellow cells, the earth-
_ .. worm has excretorv organs which, like those

ot the frog, consist ot tubes with walls that are
glandular and excretory and richly supplied with blood,
vessels ; but the tubes, instead of being collected into-
compact kidneys, are distributed along the body, one pair
to each somite, except the first three and the last. Each
tube or nephridium is thrown into loops, bound together
by connective tissue containing blood vessels. The nephri-
dium begins as a flattened, kidney-shaped funnel or nephro-
stome hanging from the front side of a septum near the
nerve cord. The nephrostome has an overhanging lip
which consists of a large crescentic central cell with a row
of marginal cells around it. This lip is ciliated. The
lower lip is not ciliated. From the funnel there leads
a narrow tube, ciliated on its sides. This passes through
the septum to the main part of the nephridium, which
lies behind the septum, in the coelom of the next somite,
opening to the exterior by the nephridiopore in that
somite. The narrow part of the tube is long and winding.

268

MANUAL OF ELEMENTARY ZOOLOGY

and loses its cilia in places. It is followed by a wider,
short, brown region, ciliated throughout, this by a still
wider tube which is not ciliated, and finally a short,
very wide, muscular tube leads to the nephridiopore. The
whole tube, except the muscular region, is formed of
hollow cells shaped like drain pipes and lying end to end.

Fig. 187.— The nephrostome or funnel of a nephridium of the earth-
worm. A , seen from in front as a transparent object ; B, in side
view, opaque, semidiagrammatic, and without its cilia.

ceiix., Central cell ; deb., debris of coelomic corpuscles and excretory granules which
is probably not able bo enter the funnel ; lower lip of opening ; m.c., marginal
cells; p e superficial layer of the peritoneal epithelium; p.e.', thickened deeper

aZeu°: the Same ; *•’ point at which the marginal cells join the lining of the tube
which turns over round the opening.

The cilia set up a stream of coelomic fluid which carries
off the substances excreted by the walls of the tubes and
probably also fine granules of excreta which it brings from
the coelom. It is not "known whether water is conserved
by. reabsorption in the nephridium of the earthworm as
it is In the excretory tubules ol many other land animals
79)* The principal nitrogenous substance excreted

THE EARTHWORM

269

is urea. Earthworms have no special respiratory organs ,
but an interchange of gases with the air takes place in the
skin, which is richly supplied with blood vessels.

The blood of an earthworm is red owing to the presence
in it of haemoglobin, which is in solution, not
in corpuscles. Colourless corpuscles are also
present. The blood-vascular system is very complicated.

Blood Vessels.

Fig. 18S. — A diagram of a nephridium of the earthworm.

br.t., Brown, ciliated tube ; m.t., muscular tube ; n.c.t., glandular, non-ciliated tube ;
n.t., narrow tube, ciliated in parts ; nst., nephrostome ; sep., septum ; ves.tiss.,
connective tissue with vesicular cells and blood-vessels ; 1, 2, 3, the three hanks
of the tube.

For details of structure see Fig. 186.

Its main outlines are as follows. A large dorsal vessel
runs the whole length of the body from the hinder end to
the pharynx. It is contractile, and in it the blood is driven
forwards. It receives blood by many small vessels from
the intestine and by two larger vessels in the tenth somite
from the oesophagus, and ends in front by breaking up
into branches which supply the pharynx. In each of the

27 o

MANUAL OF ELEMENTARY 'ZOOLOGY

somites 7-1 1 it gives off a pair of large contractile vessels or
pseudo-hearts . Ihese encircle the oesophagus and join a
ventral or submtestinal vessel which hangs by a mesentery
below the gut. In the pseudo-hearts the blood flows
downwards from the dorsal to the ventral vessel, and in
the latter it flows backwards and forwards from the region
of the hearts. From the ventral vessel the blood passes
by a series of small vessels to the intestine, and by parietal
vessels to the nephridia and to the body-wall. From these
organs, it is returned along various paths to the dorsal
vessel. Among the subsidiary vessels are a subneural and

oes hi.

s.i.v.-

ds.v-
v.n.c-
S.nv.
bw. ■

par.v.

ap/.nv.

Fig. 189. A diagram of the blood-vascular system of the earthworm.

aff.i.v Afferent vessels of the intestine ; aff.n.v., afferent vessels of the nephridia •
hA’ body;wa11 ’ d.b.u., dorsal blood vessel; d.s.v. dorso-subneural vessel-
efj.b.w.v., efferent vessel from body-wall ; eff.i.v., efferent vessel from intestinai
wall ; ht., pseudo-hearts ; mt., intestine ; oes., oesophagus ; par.v., parietal
vessel ; s.i.v., submtestinal vessel ; s.n.v., subneural vessel ; v.n c ventral
nerve cord. ’

A simpler form of this diagram will be found on p. 742.

two lateral neural vessels , m which the blood flows back-
wards, and dor so-subneur al vessels , a pair m each somite
of the intestinal region of the body, which carry blood to
the dorsal vessel from the subneural vessel, the nephridia
and the body-wall. The main blood vessels of the earth-
worm cannot be distinguished into arteries and veins, but
their ends are joined by capillaries. The dorsal vessel
and the pseudo-hearts are provided with valves which
keep the blood flowing in the proper direction.

Earthworms are hermaphrodite, every individual having
Reproduction. a complete set of organs of each sex. The female
organs include the ovaries, oviducts, and sper-
mathecae. The ovaries are two small, pear-shaped bodies

THE EARTHWORM

271

hanging into the coelom of the thirteenth somite from the
septum in front of it. Each ovary is a local thickening of
the coelomic epithelium. The broad end of the pear is
attached to the septum and contains a fused mass of unripe
ova. Ova fall from the stalk into the coelom and are taken
up by the oviducts, which lead by wide funnels from the
coelom in the thirteenth somite, pass through the septum
behind, and open to the exterior in the fourteenth. In
the latter somite, each bears a swelling, the receptaculum
ovorum or egg

db

sac, in which
the eggs are
stored and
maturation divi-
sions take place.

The spermaihecce
are two pairs of
small, round sacs
which lie in the
ninth and tenth
somites and open
in the grooves
behind them.

Their function is
to receive sperm
from another
worm. The male
organs consist of
testes, vesiculae
seminales, and
vasa deferentia.

These testes are two pairs of small, flat, ifnger-lobed
bodies attached to the hinder side of the septa in front
of somites 10 and n. Like the ovaries, to which they
correspond in position, they are local thickenings of
the coelomic epithelium. The testes bud off cells known as
sperm-mother-cells , which give rise to spermatozoa in the
vesiculae seminales. The latter are large sacs, tormed by
the walling-off of parts of the coelom, which enclose the
testes. Each consists of a median part and lateral horns.
The anterior vesicula seminalis, in somite 10, has four

s.n.v s.z.v.

Fig. 190. — A diagram of a transverse section
of the earthworm in the intestinal region to
show the arrangement of the blood vessels.

ejf.n.v., efferent vessel from nephridium ; I.n.v., lateral
neural vessel ; nph., nephridium ; other lettering as
in Fig. T89.

Fig. 191. -The development of the spermatozoa ot the earthworm.

A, Stages from the vesicula seminaiis of a young worm ; B, from that of an older
worm.

1, Sperm mother-cell ; 2-7, stages in the division to form spermatozoa ; 7-1 1, shaping
of the spermatozoa, which are still adherent to the mass of residual protoplasm
(cytophore) ; 12, a ripe spermatozoon, unstained. The head in 12 is represented
rathet too broad.

The dark bodies are the nuclei, stained.

272

THE EARTHWORM

273

lateral horns, two in front and two behind, which push out
the septa and bulge into the ninth and eleventh somites.
The posterior vesicula seminalis, in somite 11, has only
two such horns, which project into the twelfth somite.
Each sperm-mother-cell forms by multiple fission, in the
course of which the usual reduction division takes place,
a mulberry-like mass (Fig. 191), consisting of little cells
attached to a central mass of
residual protoplasm known as the
cytophore , by which they are
nourished. The little cells be-
come pear-shaped, with the broad
ends on the cytophore, gradually
increase in length, and change
their shape till the mulberry has
become a tuft of threads, each
thread being a spermatozoon with
a very slender head. Finally the
spermatozoa break loose. In the
median part of each vesicula semi-
nalis, directly behind the testes, is
a pair of large ciliated funnels with
folded walls, known as sper??i
rosettes. These funnels lead into
the vasa efferentia, of which the
two on each side join and pass
back as a vas deferens to open on
somite 15. The cilia of the rosettes
draw the ripe sperm into the
ducts.

Pairing takes place at any time f IG- i92.-~Oneofthe ovaries
from spring to autumn in warm, °f an earlhworm-
damp weather. Two worms

stretch themselves out of their burrows and place their
ventral sides together with the heads pointing in opposite
directions, their bodies being held together by a substance
secreted from the clitella. Sperm is passed from the vas
deferens of each worm, along a temporary groove, into the
spermathecas of the other, after which the worms separate.

1 he eggs are laid in a cocoon, which, secreted by the
elitellum as a broad band round the body, is passed for-

274

MANUAL OF ELEMENTARY ZOOLOGY

wards over the head. The cocoon contains a nutrient fluid.
While it is still on the clitellum eggs are passed back to it
along a temporary groove from the oviducal opening, and

a.v.s'

r.ov. od.

Fig. 193. — A dissection of the reproductive organs of an earth-
worm. The dissection is made from above, and the median
parts of the vesiculse seminales have been opened on the
right-hand side.

a.v.s., Anterior vesicula seminalis ; a.v.s' ., horns of the same ; nph.,
nephridium ; od., oviduct ; ov., ovary ; p.v.s., posterior vesicula

seminalis ; p.v.s.' , horns of the same ; r.ov., receptaculum ovorum (the
funnel of the oviduct lies immediately in front) ; sp., spermathecae ;
sp.r., sperm rosettes (funnels of the vasa efferentia) ; t., testes ; v.d.,
vas deferens ; v.eff., vasa efferentia.

as it passes the spermathecal openings, sperm received
from another worm is squeezed into it. In passing over
the head the ends of the elastic cocoon close, and it becomes
a small, lemon-shaped body, which is left in the earth.

THE EARTHWORM

275

Each cocoon contains eight to sixteen ova, which are
fertilised in it, but usually only one completes development.

Earthworms have an extensive power of regeneration.
Regeneration. thouKh it is not so great as that of Hydra. If
the body be cut in half, the head end will grow a
new tail, and the tail end, though more slowly, a new head.

In comparing the body of an earthworm with those of
the other examples of the Metazoa that we
Mesoderm, d have studied, it will be seen that in one respect

Hsmocceie, of importance it resembles the frog rather than
Hydra. The body of Hydra consists of two
epithelia — -the ectoderm and endoderm — with only a
structureless lamella between them. In a frog, a flat-
worm, or an earthworm these epithelia reappear as the
epidermis and the lining epithelium of the gut, but be-
tween them is a great mass of tissue which comprises the
skeletal tissues, muscles, excretory and generative organs,
and so forth. These tissues are together known as the
mesoderm , and animals which possess this third laver are
known as Triploblastica , whilst those, like Hvdra , which
possess only two are Diploblastica. Both in the earth-
worm and in the frog the mesoderm contains cavities of
two kinds, the primary body cavity or hcemoccele or blood
vessels , and the “ true ” or seco7idary body cavity or coelom.
The functions of the coelom are threefold, (i) It forms a
perivisceral cavity , which surrounds the principal viscera
and so gives room for their movements. (2) From its walls
are derived the generative cells. This is clearly seen in the
case of the ova, which are shed into the perivisceral cavity,
but it is less clear in the case of the spermatozoa, because
these are developed in special vessels, derived from the
coelom but closed off. (3) It is concerned in excretion. In
an earthworm, where the yellow cells of its walls form
excreta which are expelled from the body, this is more
obvious than in the frog, but the kidney tubules of the
latter — which are not nephridia but ccelomoducts (see
below) — in the tadpole (p. 635) are, like the nephridia of
the earthworm, open to the coelom, and draw thence a
fluid which contains substances that are excreted. The
haemocoele is a system of spaces of more complex form
than the coelom, and rarely (p. 298) perivisceral. Its function

276 MANUAL OF ELEMENTARY ZOOLOGY

is to contain the blood and lymph. A blood-vascular system
is a means of transport made necessary in most triploblastic
animals by the presence of the great mass of internal tissues
which constitute the mesoderm.

The mesoderm necessitates not only means of transport
within it but also means whereby materials
cSiomlduetol may be conveyed from it out of the body.

There are two principal types of organ which
have this function — the nephridium and the ccelomoduct.
The nephridial system arises by ingrowth from the ecto-
derm and consists of tubules, usually fine and branched,
the branches ending in cells where a tuft of flagella hang
into it. In this condition it constitutes the flame-cell
system of Platyhelminthes. Here it is imbedded in
parenchyma but when it occurs in animals with a coelom — -
in the annelid worms of which the earthworm is one, and
the lancelet ( Amphioxus ) — it lies in that cavity. It then
consists of a series of separate nephridia. These may — -
as in some of the worms and in A mphioxus — end blindly
in cells called solenocytes which differ from the flame-cells
of the Platyhelminthes in having a longer neck and fewer
flagella. In other cases, as in the earthworm, each
nephridium opens to the coelom by a multicellular funnel—
the nephrostome. The nephridial system is primarily an
apparatus for the removal of excess water (if necessary)
and of excreta. Coelomoducts are mesodermal tubes,
often of considerable diameter, which in typical instances
open by a funnel from the coelom and lead thence to the
exterior, one pair in each somite if the animal be seg-
mented. Their primary function is perhaps the removal
of gametes which, as we have seen, are developed on the
wall of the coelom, but they often usurp the excretory
function of the nephridia. In the earthworm most somites
possess nephridia only but those in which the gonads lie
have also coelomoducts in the form of oviducts and vasa
deferentia. In certain annelid worms each nephridium
has united with the adjacent ccelomoduct to form a
nephromixium , in which the funnel of the ccelomoduct
opens through the duct of a nephridium. In others (as in
Nereis described below) the ccelomoduct does not open
and is reduced to a patch of cilia on the ecelomic wall.

THE EARTHWORM

2 77

Another feature of the morphology oi an earthworm to
which attention must be called here is its
segm a segmentation. We have seen that merism or
the repetition of parts is universal among animals. In an
earthworm the whole body consists of similar divisions
(the somites) arranged one after the other in a line or
series. Each division contains a ring of the body-wall,
with chaetae and openings, a separate portion of the
coelom, a section of the gut, a ganglion, nephridia, and

Fig. 194 — Nereis cultrifer. — From Thomson.

a., Anus ; a.c., anal cirri ; c., tentacular cirri ; e., eyes ; />., palp ;
fie.. periM. mium ; t.. tentacles.

blood vessels. A body so constructed is said to be meta-
merically segmented. Most of the somites resemble one
another closely, but in the forepart of the body they show
considerable differences in the reproductive, alimentary,
and other organs. A modification of the foremost somites
to form a head (cephalisation) is, as we shall see, a con-
spicuous feature of some animals, but it can hardly be
said to exist in the earthworm. The body of an earth-
worm is actually composed of similar divisions, because •
all the organs are repeated together at regular intervals.
There are other animals in which only some ot the organs

278 MANUAL OF ELEMENTARY ZOOLOGY

are thus repeated, as in the frog where the vertebrae,
nerves, and to some extent the muscles exhibit segmenta-
tion, so that the regions of which the body might be
regarded as composed are much less distinct than they
are in the earthworm. In such cases segmentation is said
to be incomplete. The tapeworm presents an example of
a kind ot segmentation, known as strobilation , which is
very complete, but is of quite a different nature from that

Fig.. 1 95- T transverse section through Nereis cultrifer , slightly
simplified. The parapodia are shown in perspective. ' Magnified.
— After Shipley and MacBride, with modifications.

I> Cuticle ; 2, epidermis ; 3, circular muscles ; 4, longitudinal muscles ; 5, oblique
muscles forming a partition ; 6, somatic layer of peritoneal epithelium ; 7, coelom *
8, splanchnic layer of epithelium ; 9, cavity of intestine ; 10, dorsal blood
J1> ventral blood vessel; 12, ventral nerve cord; 13, nephridium
ii» section ; 14, ova ; 15, notopodium ; 16, neuropodium ; 17, dorsal cirrus ;
l'8, ventral cirrus ; 19, chadae ; 20, aciculum ; 21, muscles which protrude the
acicula, and thus the noto- and neuropodium ; 22, ciliated organ (vestige of
ccelomoduct).

of the earthworm, the youngest segments being at the
front end and the old ones becoming independent and
dropping off, whereas in the earthworm the youngest
segments are those at the hind end and the segments are
not shed but are integral and necessary parts of the body.

The term segmentation is used in zoology to denote very various
phenomena. Its primary meaning is the division of an object into
parts. Such parts are segments, and when they are arranged in
linear series the segmentation is said to be metamerzc . The term is
applied to the division of the whole body, of parts of it, such as limbs.

I

ANNELIDA : NEREIS

279

or of the ovum. When it is used to indicate that the whole body of an
organism is composed of integral parts, metamerically arranged the
segments are known as somites ; when a limb is similarly composed
its segments are podomeres . When an organism reproduces asexually
by breaking off successive portions of itself in a linear series such
segments are known until they separate as strobilce. The segments
into which an ovum divides are blastomeres.

The earthworm is adapted to a burrowing habit and a
Nereis. vegetarian diet. Many marine worms, how-

ever, while they resemble the earthworm in
most respects, lead a free and predaceous existence. Of

Fig. 196 — The head of Nereis , with the pharynx protruded.

e., Eyes ; j., jaw ; p., palp ; pe., peristomium (first two segments, fused) ; ph.t
pharynx; pp., first ordinary parapodium ; pr., p~ostomium ; t., accessory teeth ;
tc., tentacular cirri ; te., tentacle.

these, Nereis cultrifer , common under stones on the south
coast of England, where it is known as the Red Cat and is
used as bait, is a good example. The body of this worm is
about six inches in length, of a greenish colour, with red
on the limbs and where the dorsal blood vessel shows
through, roughly cylindrical, tapering towards the hinder
end, and divided into about eighty somites. Like the
earthworm, it is covered with a thin cuticle and provided

28o

MANUAL OF ELEMENTARY ZOOLOGY

with chaetae, but the chaetae are longer and much more
numerous than in the earthworm, and are borne on
movable limbs or parapodia , of which a pair is placed on
each somite. A parapodium is a flat, hollow vertical
process of the body-wall, standing out at the side of its
segment and serving to pull the animal along in creeping,
or to row it in swimming. It is cleft into two principal
lobes, a dorsal notopodium and a ventral neuropodium .
Each of these is again divided into smaller lobes and
bears at its base a slender process known as a cirrus. A
stout, deeply embedded chaeta or aciculum supports the
notopodium and another the neuropodium. The front
end of the body is modified to form a definite head. This
consists of the prostomium and the peristomium. On
the prostomium are situated dorsally a pair of prostomial
tentacles and two pairs of eyes, each of which is a pit lined
by pigmented cells and enclosing a gelatinous mass which
serves as a lens. Ventrally the prostomium bears a pair of
stout palps. The peristomium carries on each side two
pairs of long, slender tentacular cirri , and probably
corresponds to two fused somites. A bilaterally sym-
metrical animal which leads an active life always has a
head, and if the animal be segmented there is a tendency
for the foremost somites to enter into the composition of
the head. This is known as cephalisation. Behind the last
segment is a conical region without parapodia which bears
a pair of slender anal cirri and the terminal anus.

The musculature of Nereis is more complicated than that
of the earthworm, the longitudinal muscle fibres being
grouped into four powerful longitudinal bundles, two
dorsal and two ventral, while there are oblique muscles to
move the parapodia. As might be expected from the
better provision of sense organs on the head, the brain
also is more complex. The alimentary canal is simpler
than that of the earthworm, but the pharynx can be caused
to protrude by being turned inside out, and is lined with
cuticle, thickened in places to form numerous small teeth
and a pair of strong jaws with which the prey is seized.
The sexes are separate. The reproductive organs are very
simple, consisting of temporary masses of cells, which
arise from the coelomic epithelium. The ova and sperm

ANNELIDA : NEREIS

281

probably escape by temporary openings formed in the
body-wall, and fertilisation takes place in the water. The
free young are at first
very unlike the parents,
being minute, globular
creatures, known as
trochospheres , which

swim by means of a
girdle of cilia in front of
the mouth and have an
apical tuft at the front
end. They undergo a
gradual change into the
adult, becoming oval
and then lengthening FlG- I97*— A. The trochosphere of
and segmenting. Their Nerets. Modified, after Wilson.

mesoderm is formed as Apical tuft of cilia; eye; m., opening

of mouth; mes., mesoderm; pr., preoral
ring of cilia; stm., stomodasum (the pouch
of ectoderm which forms the mouth and
gullet).

two ventro - lateral
bands, each thrown off
by the continual division
of a pole cell at the
hinder end, and spreads
round the gut, between
ectoderm and endoderm,
the coelom appearing in
it. The larva of Nereis
is not in all respects a
typical trochosphere. In
Fig. 197, B a more
typical example is
shown, in a later stage of
development than that
represented in Fig. 197,

A. It has between ecto-
derm and endoderm a 3
large space (blastocoele)
which is lacking in the

trochosphere of Nereis. av.y Anus; an.c., anal tuft of cilia; ap.c.,

The leeches are apical tuft of cilia; m., mouth; muse.;
, " larval muscles ; nph., larval nephridium •

another group of seg- Pr-> Preoral ring of cilia ; pt., postoral

mented worms related to ta&i SJ"6 beeiming *°

CLTL.

A typical trochosphere in an early
stage of the transformation into
the adult.

282

MANUAL OF ELEMENTARY ZOOLOGY

The Leech.

the earthworms. Hirudo medicinalis , the Medicinal Leech,
a dweller in freshwater pools, marshes, and
sluggish streams, is found sometimes in this
country but more commonly on the Continent, where,
when it was more used in medicine than at present, it
was bred in large numbers in special ponds. It lives
normally by sucking the blood of frogs and fishes, but also,
when it is full grown, on that of warm-blooded animals
which enter its haunts, and it will feed on man, though to
induce it to do so his skin may have to be moistened with
blood or milk or pierced by a small cut. An active specimen
will draw one or two drams of blood. The body of the
leech is 3-5 inches in length, somewhat flattened, and pro-
vided at each end with a downward-facing sucker. It is
encircled by 95 minute rings or annuli , and brightly marked
in various shades of green, yellow, and black, paler below
than above. The annuli do not indicate the true segmenta-
tion. In the greater part of the body five of these lesser
rings go to a somite, but towards the ends there are fewrer,
and in the head or region of the anterior sucker (prostomium
and first five somites) there are eleven annuli, while the
posterior sucker represents seven somites fused without
annulation. Unlike that of the earthworm or Nereis , the
number of somites is a definite one, amounting in all to
33, including those of the head and hinder sucker. The
mouth lies in the midst of the anterior sucker, and the anus
is a minute opening above the base of the hinder sucker.
The male and female genital openings are median on the
second annuli of the 10th and nth somites respectively.
O11 the last annulus of each somite from the sixth to the
twenty- second, are the openings of a pair of nephridia.
On the first annulus of each somite is a transverse row
of minute sense-papillae. On the head a pair of these
in each somite are transformed into minute eyes, recog-
nisable by their pigment as dark spots. There are no
chaetae. The worm can walk by looping like a looper
caterpillar, and swim by undulation of its body.

The body is covered by a thin cuticle, which is shed
from time to time. Under this lies an epidermis, between
the bases of whose cells run blood capillaries, so that the
skin is a respiratory organ, in which the blood is exposed

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284 MANUAL OF ELEMENTARY ZOOLOGY

r-cr.i

si*

-■cr.ii.

to the surrounding water. Below stand circular and
longitudinal muscle-layers, and within these a layer of
botryoidal tissue , composed of branched canals, whose
walls are laden with black pigment, while their cavity is
lull of blood. This tissue takes the place of a peri-
visceral cavity and imbeds the gut.
The mouth is provided with three
jaws, which are compressed cushions
of muscle covered with a finely
toothed layer of cuticle : by these
the skin of the prey is pierced with
a characteristic tri-radiate wound.
A muscular pharynx succeeds the
mouth ; it pumps the juices of the
prey, and into it open numerous
unicellular glands whose secretion
prevents blood from clotting so
that the leech’s food remains fluid
while it is being taken and passed
backward to be digested. It is
owing to this secretion that bleeding
continues for some time after the
leech is removed from its wound,
and an extract of the heads of
these worms is sometimes used in
physiological experiments to pre-
vent clotting. After the pharynx

medicinal leech, as Stands the very lar§e croP (segs-
seen when it is not 8-18) with eleven pairs of lateral

caeca, of which the last is much
larger than the rest and extends
backwards on each side of the
remainder of the alimentary canal.

This consists of the stomach — a
narrow tube, with an enlargement at its start and a
spiral fold of its inner wall — the intestine, narrower than
the stomach, and the rectum, somewhat wider. The
blood sucked for food is stored in the crop, whose caeca
are more or less dilated according to the amount of
their contents, and passed drop by drop into the stomach,
where it at once turns green and then is digested. A

mtr

V

\J

Fig. 199. — A diagram-
matic view of the ali-

gorged with blood.

cr.i., cr. 11., Caeca of the
crop; int., intestine;
ph., pharynx ; rtn., rec-
tum ; si., stomach.

ANNELIDA : THE LEECH

2S5

full meal will last the animal for several months or a
year. Seventeen pairs of nephridia lie in somites 6-22.
Each is a mass of glandular tissue traversed by a system of
intracellular ductules. There is no internal opening, but
those which lie in the testis somites have a swollen end
lying in the capsule ol the testis, and bearing a number
of ciliated funnels which do not communicate with the
ductules. There are two svstems of tubes which ccntain

Fig. 200. — A transverse section of the medicinal leech. — After Bourne.

b.t., Botryoidal tissue; c.m., circular muscles; cap., capillary; cr., crop; cr'., a
caecum of the same; c#.," cuticle ; dm., dorsoventral m,uscle ; d.s., dorsal
sinus; ep., epidermis ; g.c., gland cells ; l.nt ., longitudinafmuscles ; lateral

vessel ; n.c., nerve cord ; t., testis ; t.s., testis sinus, with end of nephridium ;
v.d vas deferens.

a fluid like blood — a red plasma with a few colourless
corpuscles. One of these systems is the true blood-vascular
system : its principal vessels are two lateral trunks, which
unite before and behind, and are connected also by a net
work of capillaries. There are no hearts, but the lateral
vessels are contractile. The other vascular system consists
of a dorsal and a ventral longitudinal sinus, which are also
connected before and behind and by means of a capillary
system. The walls of this system are thinner than those

286

MANUAL OF ELEMENTARY ZOOLOGY

of the true blood vessels. They represent the coelom, as
may be seen from the fact that the ventral sinus encloses
the nerve cord and communicates with capsules around the
ovaries and testes. The botryoidal tubes are also in
communication with the sinus system. It is said that
the capillaries of the true blood system are connected
with it. The nervous system is of the same type as those
of the earthworm and Nereis. A small brain, above and
before the pharynx, is connected by a pair of very short
circumpharyngeal commissures with a ventral cord which
•carries at wide intervals twenty-three ganglia. Of these
the first or subpharyngeal and the last each represent
several fused. Nerves are given off from the brain and the
ganglia. The commissures which unite the ganglia of
the ventral cord are seen in section to be double, with a
slender median strand.

The animal is hermaphrodite. There are nine pairs of
testes, enclosed in spherical capsules in the i2th-2oth
somites, with on each side a common vas deferens, which
is coiled as an “ epididymis ” in the ioth somite, where
the vas deferentia join to open on a muscular, protrusible
penis, surrounded at its base by a “ prostate ” gland. The
single pair of ovaries lies in the nth somite, in which
its short oviducts open by a common vagina. The eggs
are laid in cocoons secreted by “ clitellar ” glands in the
skin of the ioth-i2th segments and placed in holes made
in the banks, above water. The young resemble the
parents and feed at first on the juices of water insects, and
the like.

Segmented, ccelomate animals, with a thin cuticle, a
Annelida. closed blood-vascular system, nephridia, and
a nervous system on the same plan as that of
the earthworm, are known as Annelida. The earthworm,
Nereis , the lug-worm ( Arenicola ), which is often used for
bait, and leeches belong to this group.

CHAPTER XV

THE CRAYFISH. ARTHROPODA

Crayfishes are found in many English rivers, especially in
those which rise in chalk or limestone hills.
Habits. They are little, lobster-like creatures, which

make burrows in the river banks. They dislike strong
light and during the daytime generally remain in their holes
with only their pincers and long feelers projecting. When
they come out they crawl stealthily about, searching con-
stantly for their food, which consists of organic matter
of any kind, plant or animal, dead or alive, that they
are able to seize and break up with their pincers. II
danger threatens, they dart backward suddenly and swiftly
They are used for food, especially for garnishing salads,
and were formerly caught in large numbers in this country
by means of wicker crayfish-pots, but in 1887 their numbers
were greatly reduced by a disease, and at present crayfishes
for the table are imported from the Continent.

The English crayfish, Astacus torrenlium, is about three
inches long, and of a dull, greenish colour,
Feature* which harmonises well with the surroundings
in which it lives. The species imported from
the Continent is A. fluviatilis , which is larger and has red
colouring on the pincers and legs. The body of a cray-
fish is armoured with a thick chitinous cuticle, strengthened
by salts of lime, which in places is thinned to form joints :
it is segmented, each segment (somite) bearing a pair of
jointed limbs, but in the front part the segments are fused
to form a fore-body or cephalothorax , where the only con-
spicuous sign of their existence is the presence of several
pairs of limbs, though parts of the armour and certain in-
ternal organs are also segmentally arranged. The rest of the

288

MANUAL OF ELEMENTARY ZOOLOGY

body, known as the hind body or abdomen , is more com-
pletely segmented. At the end of the abdomen is a flat
piece known as the telson, on the under side of which
the anus opens. The telson bears no limbs, and is divided
by an imperfect transverse joint. The armour of each
segment of the abdomen consists of a broad back-piece or
tergum and a narrow belly-plate or sternum , with a pair of
V-shaped prolongations, known as the pleura , joining them

at the sides. There are no
pleura on the first abdominal
somite. The tergum, ster-
num, and pleura of each som-
ite form a continuous ring.
The limbs are jointed to the
hinder side of the sternum near
its outer ends, and the part
of the sternum between each
limb and the adjoining pleuron
is sometimes called an epi-
meron . The terga overlap one
another from before back-
wards and slide over one
another as the abdomen is
straightened and bent, the
armour of each somite being
joined to that of the next by
thin cuticle, which allows of
movement. In the cepha-
lothorax- the terga are fused
to form a shield or carapace .
This is prolonged in front into
a beak-like rostrum and is
crossed by a furrow, which is called the cervical groove be-
cause it is supposed to mark the separation of two regions
known as the head and thorax. At each side of the body
a fold of the carapace overhangs as a lean-to roof, the gill
cover or branchiostegite, which encloses between itself and the
side of the body a chamber in which the gills lie. Behind the
cervical groove a branchiocardiac groove on each side marks
off the branchiostegite from a median cardiac region which
roofs the thorax. The cuticle of the inside of the brand*

Fig. 201. — View of a crayfish
from above. — After Huxley.

Note cervical groove, cardiac region of
carapace, rostrum, and tail fan.

THE CRAYFISH

289

iostegite and part of the side of the body underneath it are
thin. On the ventral side of the cephalothorax the limb'
of each pair are close together, but small sterna lie between
them. The head is, of course, the region which contains
the mouth and the principal sense organs. The mouth is
placed on the ventral surface at some distance from the
front end, and in front of it the sternal surface slopes up-
wards to the rostrum. At the sides of the latter, upon a
pair of short, movable stalks, are placed the eyes, and below
these stand two pairs of feelers or antennae.

The limbs or appendages number nineteen pairs, without
counting the eyes, which are by some authorities
Limb*. reckoned as limbs. We shall not take this

view, but as there is evidence in the development of the
crayfish and of related animals that the foremost region of
the head corresponds to a somite, we shall regard the
body as containing twenty somites, of which the foremost
bears no limbs. The telson is not a somite. Of the twenty
segments, the first six form the head, the next eight the
thorax, and the last six, with the telson, the abdomen.
The parts of which a complete limb consists are best
seen in the limbs known as the third pair of maxillipeds
(Fig. 205, A), which lie immediately in front of the great
pincers. In each of these we may distinguish a two-jointed
basal region or protopodite , the segment nearest the body
being known as the coxopodite , the next as the basipodite.
On its outer side the coxopodite carries a large, flat, thin-
walled structure, known as the epipodite , bearing many
small finger-processes, which constitute a gill. At the
base of the epipodite is a knob bearing a tuft of threads
known as the coxopoditic setce or setobranch. The basi-
podite bears two structures. From its outer side arises a
slender, jointed appendage known as the exopodite. At
its end, continuing the axis of the limb, is the stout, five-
jointed endopodite , whose segments (podomeres), starting
from the basipodite, are known as the ischiopodite , meropo-
dite , carpopodite , propodite and dactylo podite. The other
limbs are built upon the same general plan as the third
maxilliped, but in detail the structure of every one of them is
adapted to the particular work it has to perform, each part
being of a different shape in each pair of limbs, or sometimes

19

290

MANUAL OF ELEMENTARY ZOOLOGY

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THE CRAYFISH

291

nil. 6, Sixth abdominal limb; at. 1, antennule ; at. 2, antenna; lr., branchiostegite ; c.g., cervical groove; ch., cheliped ;
crd., cardiac region of carapace; en., endopodite ; ex., exopodite ; mxp. 3, third maxilliped ; //., pleuron ;
protopodite ; rst., rostrum ; sc. a., scale of antenna ; telson ; tg., terga ; w.l. 4, last walking leg.

292

MANUAL OF ELEMENTARY ZOOLOGY

absent altogether. The first two pairs are sensory, and have
long lashes for searching the surroundings of the animal.
The next six pairs are jaws, used for bringing the food to
the mouth and chewing it : these are short and stand
close behind the mouth. Then come two great shears

or pincers which are
the principal grasping
organs, next four pairs of
legs, and then the six
pairs of! limbs of the
abdomen, of which some
are concerned in repro-
duction, some help the
animal forward by pad-
dling while it is walking,
and the last pair are used
in rapid swimming. Ex-
opodites are wanting
from the legs, great pin-
cers, and first two pairs
of jaws, and epipodites
are found only upon the
limbs of the thorax.

The first limb of each
side is known as the an-
tennule or first antenna.
It is peculiar in having
three, instead of two,
segments in its protopo-
dite. The first segment 1
is large and three-sided :
upon its upper side there
opens, by a slit edged
with bristles, the stato-
cyst, which will be de-
scribed later. The third segment bears two many- jointed
lashes or flagella. These are often compared to the
exopodites and endopodites of the other limbs, but
the comparison is probably not justifiable. The outer
lash bears on the under side of most of its segments certain

1 See the note opposite.

Fig. 204. — The antennule and
antenna of a crayfish.

A, External view of the antennule of the
left side ; B, ventral view of the
antenna of the right side.
bp., Basipodite ; cp., coxopodite ; en.,
endopodite ; ex., exopodite ; o.g.,

opening of the green gland.

THE CRAYFISH

293

peculiar bristles which are supposed to serve the sense of
smell. The second limb is the antenna (or second antenna).
Its coxopodite is short and wide and bears below a knob,
upon which opens the green gland or kidney. The bash
podite bears the endopodite on the1 median side. The
exopodite is a flat, triangular, pointed scale , and the

Fig. 205. — The mouth appendages of the left side of a crayfish.

A, Third maxilliped ; B, second maxilliped ; C, first maxilliped ; D, maxilla (second
maxilla) ; E, maxillule (first maxilla) ; F, mandible in ventral view ; G, the same
in dorsal view, somewhat enlarged, showing molar process.

bp., Basipodite ; cp., coxopodite ; crp., carpopodite ; dtp., dactylopodite ; en.,
endopodite; ep., epipodite ; ex., exopodite; gill; isp., ischiopodite ; mrp.,
meropodite ; pr., protopodite ; prp., propodite ; sb., setobranch or tuft of coxo-
podite setae ; 1-5, joints of endopodite.

endopodite is a very long flagellum. The third limb or
mandible has a large, broad, and very strong coxopodite 1
with a toothed incisor edge which bites against that of its

1 Perhaps really a joint known as the precoxa , which corresponds to
the first joint of the antennule and is in the other limbs obscured or
absent. There is reason to believe that the true coxopodite of the
mandible is absent. In the maxillule the precoxa and coxopodite
appear to be fused. In the second maxilla the precoxa may be
represented by the first lobe of the protopodite, and in the thorax
the precoxa of each limb is absorbed into the side ol the body. There
is no evidence as to its identity in the abdominal limbs.

294 MANUAL QF ELEMENTARY ZOOLOGY

'►i " >

lellow on the other side of the body. Above the incisor
edge, and therefore hidden in ventral view, is a broad,
irregular ridge known as the molar process. The basi-
podite and an endopodite of two segments form a small
three-jointed appendage or palp in front of the coxopodite.
The mandibles lie at the sides of the mouth. Behind it
come the limbs of the fourth pair, known as the maxillules
or first maxillae. Each of these consists of three thin
plates joined to a small basal piece. One plate is an
expansion of the coxopodite,1 the second represents the
basipodite, and the third the endopodite. The fifth limb is
the maxilla (or second maxilla). It is\ a flat structure,
deeply cleft into several parts. The protopodite bears two

ep.

Fig. 206. — The first walking leg of a crayfish. Letters as in Fig. 205.

thin lobes directed towards the middle line of the body and
each in turn divided into two.2 The endopodite is a narrow
structure directed forwards. The exopodite is a wide plate
projecting backwards and forwards from the outer side of
the limb and known as the scaphognathite. The second
maxilla lies under the front end of the branchiostegite, and
the function of the scaphognathite is to set up a current of
water over the gills by bailing it forwards out of the gill
chamber. The sixth limb, or first maxilliped , is the first

1 See note to p. 293.

2 The first double lobe probably represents the precoxa and
coxopodite ; the two parts of the second lobe probably belong to the
basipodite and the segment which succeeds it.

THE CRAYFISH

295

of those which belong to the thorax. Two broad lobes
represent its coxopodite and basipodite, the endopodite is
small and two-jointed, and the exopodite, shaped like that
of the third maxilliped, is large. The epipodite is present as
a very large plate, which does not bear a gill. The second
maxilliped is much like the third, but has a smaller endo-
podite and a relatively larger exopodite. The third maxil-
liped has already been described. Behind the maxillipeds
come five pairs of legs or pereiopoda, ot which the first, the
ninth of the whole series of limbs, bear great pincers and are
called the chelipeds , the rest being the walking legs. In each
of these limbs the exopodite is wanting and the endopodite is
long and strong and consists of five podomeres, named as
in the third maxilliped. In the chelipeds and the walking
legs of the first two pairs the propodite has a projection,
against which the dactylopodite bites so as to form a pair ot
pincers. An epipodite bearing a gill is present upon each
of the legs except the last pair. On the coxopodite ot the
second walking leg of the female is a round opening,
through which the eggs are laid, and the sperm of the male
is passed through a similar opening upon his last leg. Ot
the abdominal limbs the first and second pairs are best
studied after the third, fourth, and fifth. The latter are
alike and consist each of a short coxopodite, a long basi-
podite, and an endopodite and exopodite each composed ot
a number of imperfectly separated podomeres, of which the
first is longer than the rest. The endopodite is rather
longer than the exopodite, and both bear numerous,
plumed bristles. The second abdominal limb ot the female
is like those behind it. In the male the first podomere of
the endopodite is much elongated and bears at the end on
the outside a thin plate rolled into a scroll. The first
abdominal limb has no exopodite in either sex. In the
female it is minute. In the male the basipodite and endo-
podite are fused, flattened, and rolled scrollwise into a tube
for conveying sperm to the female. The limbs ot the last
(sixth) abdominal pair have short, undivided protopodites,
and very broad endopodites and exopodites, the former ot
one, the latter of two podomeres. They are directed back-
wards, and form with the telson the tail fan used in
backward swimming.

296

MANUAL OF ELEMENTARY ZOOLOGY

Head

Thorax

Table of the Segments of the Crayfish
1. Preantennal limbless segment

Abdomen

2. Antennules .

3. Antennae (11)

4. Mandibles .

5. Maxillules .

6. Maxillae (11)

7. Maxillipeds (1)

8. „ (11)

9- » (m)

10. Chelipeds

11. Walking legs (1)

12. „ ,, (n) 9

13* »» >» (in)

14* »> (i^)3

'15. Abdominal limljsflT)

16.

17.

18.

19.

20.

99

Telson

9 9
99

(II)

(Hi)

( v)
(v)

(VI)

9 Female opening.

j Sensory limb*.

laws.

1 ,egs.

Uniramous limbs,

r Paddles.

j-Tail fan.

<5 Male opening.

The ectoderm or epidermis of the crayfish consists of a
layer of protoplasm with nuclei, which in many
cuticle and parts is not divided into cells and is therefore
a syncytium (p. 147), though m places it
forms a columnar epithelium. Outside it, lies a cuticle
which it secretes, and, as we have already seen, this
cuticle contains chitin (p. 257), and is for the most part
thick and hardened with salts of lime, but remains thin and
flexible in certain places so as to form joints which allow the
parts of the body to move upon one another, and also in the
gill chambers. In places it bears bristles {seta) of various
shapes. These are hollow, and the epidermis is continued
into them and is here often connected with nerve fibres, so
that the bristles serve as sense organs of various kinds.
From time to time the cuticle is shed and a new one
secreted ; this allows of growth. Moulting takes place
frequently while the animal is young, but the old male
sheds its cuticle only twice a year, and the female only once.
As the time for moulting draws near, a new cuticle begins

THE CRAYFISH

297

to form under the old one, which is loosened from the
epidermis, and the crayfish goes into hiding, because the
new cuticle is soft and the animal will be helpless for some
days while it is hardening. The shell then splits across the
back and along the limbs, and the crayfish, lying on its
side, draws itself out of the old cuticle.

There is in the crayfish no continuous muscular body-wall,
but numerous muscles, composed of striped
fi b r e s,
move the
various

h. g.~

Skeleton,
Muscles, and
Locomotion.

d.ab.a.

ext.vn-

v.nc.

parts of its body, be-
ing attached to the
inside of the piecesvof
the armour. Thus
the skeleton is ex-
ternal, not, like that
of a frog, internal.

Its pieces, known as
sclerites , usually abut
upon one another
across the soft joint-
ing membranes by
hard knobs which
serve as hinges. In
the thorax ingrowths
of the cuticle pro- bp.
vide a kind of false
internal skeleton.

This has the form of a
complicated scaffold-
ing along the ventral side of the animal, and is known as the
endophragmal skeleton. In the limbs, as in those of the
frog, opposing muscles (flexors and extensors) bend and
straighten each joint. Ingrowths of the cuticle serve as
tendons for them. The abdomen also is moved by two
sets of muscles. A dorsal set of extensors starts from the
inside of the carapace and is inserted into the terga of the
abdominal somites. When they contract, these muscles
draw forward the terga and thus straighten the abdomen.
Ventrally, powerful and complicated flexors connect the

Fig, 207. — A semi-diagrammatic drawing of
a transverse section of the abdomen of
the crayfish.

Basipodite ; cp., coxopodite ; d.ab.a., dorsal
abdominal artery ; en.t endopodite ; ex., exo-
pod ite ; ext.m ., extensor muscles ; fl.m., flexor
muscles; h.g., hind-gut; pi., pleuron ; pr.,
protopodite ; tg., tergum ; st., sternum ; v.ab.a.,
ventral abdominal artery ; v.n.c. ventral nerve
cord.

298 MANUAL OF ELEMENTARY ZOOLOGY

sterna with one another and with the endophragmai skeleton
(Fig. 2 1 o). The flexors, when they contract, draw closer
the sterna and thus bend the abdomen. By this movement,
spreading at the same time its tail fan, the crayfish makes
the sudden backward jumps by which it escapes from
its enemies. Its gentle forward movements are carried
out by the walking legs, aided by a padding of the
abdominal limbs. The legs of the first three pairs pull
and those of the last pair push, and their movements are
carried out in such a way that the animal is always standing
upon six legs while two — which are on opposite sides and
of different pairs — are in motion.

The power of regeneration, though it is less in the
crayfish than in earthworms and much less
and Autotomy. than in Hydra , is still considerable. A whole
limb which is injured can be grown again.
The injured leg is first cast off by a spasmodic contraction
of some of its muscles which causes it to break through at
the basipodite, the internal cavity — which, as we shall see,
is a blood space — being here crossed by a partition which
leaves only a small opening, through which the nerves
and blood vessels pass. When the limb is cast off this
opening is quickly closed by a blood clot, after which the
cuticle grows across the wound. Beneath the scar the
new limb is formed as a sort of bud and gradually takes
shape. At the next moult it becomes free, though it is
still small, and it increases in size at each moult, until a
normal limb has been provided. This power of casting
off limbs is known as autotomy. It is sometimes used
as a means of escape from enemies which have seized one
of the limbs, but this is not so common in the crayfish as
in some animals that are related to it.

The body of the crayfish contains a spacious perivisceral
cavity, in which the internal organs lie. This
cavity and is not a coelom, but an enlarged portion of the
system tary haemocoele (p. 275;, and communicates with
the blood vessels. The alimentary canal fills
the greater part of this cavity. The mouth is an elongated
opening below the head between the mandibles. It has
in front a wide upper lip or labrum , and behind it is a pair
of lobes ( paragnatha ) known together as the lower lip or

THE CRAYFISH

299

metastoma. A short, wide gullet leads upwards into the
large prove ntri cuius, often called the “ stomach.” 1 his

Fig. 208.- — The proventriculus of the crayfish.

A The whole organ from above ; B, the same from the right side ; C, the left half from
within, the muscles being relaxed ; D, the ossicles of the mill in median section,
the anterior and posterior gastric muscles being contracted ; E, the mill m plan.
All the figures are semi-diagrammatic, much detail being omitted.

br*., Bristles for filtering ; car., cardiac ossicle ; cm., caecum ; f.c., pyloric or filter-
chamber ; glth., position of gastrolith; h.g., hind-gut ; l.p., lateral pouch; l.t.,
lateral tooth ; w.c., mill-chamber ; m.g., mid-gut ; m.t., median tooth ; o.b.,
opening of bile duct ; ces., oesophagus ; p.car., pterocardiac ossicle ; p.py., pre-
pyloric ossicle ; py., pyloric ossicle ; u.car., uorocardiac ossicle ; «-,tne several
nieces of an arrangement of valves which directs the solid residue of the food into
the hind-gut, there to become the faeces ; z.car., zygocardiac ossicle.

consists of two chambers, a large forepart or mill-chamber ,
often known as the “ cardiac division of the stomach,” and
a smaller hind part or filter-chamber , often known as the

300

MANUAL OF ELEMENTARY ZOOLOGY

“ pyloric division of the stomach,” separated from the
mill-chamber by a pit in the roof. From the filter-chamber
the short mid-gut or mesenteron leads backwards to the long
kind- gut, sometimes called the “ intestine.” The epidermis
and cuticle turn inwards at the mouth and line the gullet
and proventriculus, which are together known as the fore-
gut. The mid-gut is lined with soft endoderm, and the
hind-gut is again lined with epidermis and cuticle. Thus
the regions often called stomach and intestine in the cray-
fish do not correspond with those so named in the frog
and earthworm, being lined with ectoderm, not endoderm.
The cuticle in the gut is for the most part thin, but in
places^-in -the - proventriculus it Torms stout plates or
ossicles, certain of which bear strong teeth which project
into the forepart of the organ. By the action of muscles
these can be brought together to crush the food. The
whole apparatus is known as the gastric mill.

Two large plates lie across the roof in the two divisions, and are
known as the cardiac and pyloric ossicles. They are joined in the
middle by two smaller pieces, the urocardiac and prepyloric ossicles ,
which lie respectively in the front and hinder walls of the pit between
the two divisions. From the lower end of the prepyloric ossicle there
projects into the proventriculus the forked middle tooth. When the
mill is at rest the pit passes backwards, so that the prepyloric ossicle
in its hinder wall is also directed backwards under the pyloric, and
its tooth points backwards. At each side of the pit the cardiac and
pyloric ossicles are connected by two more pieces, the zygocardiac
ossicle , which articulates behind with the side of the pyloric, and the
pterocardiac ossicle , which joins the front end of the zygocardiac to the
side of the cardiac ossicle. These side ossicles do not run straight, but
slope outwards to meet at an angle, so that the outline of the whole
framework of the mill is roughly hexagonal. Internally each zygo-
cardiac ossicle bears a large, ribbed, lateral tooth. Anterior and
posterior gastric muscles run from the cardiac and pyloric ossicles
respectively to the carapace. When they contract they pull these
ossicles apart. The result is that (i) the upper end of the prepyloric
ossicle, being pulled backward by the pyloric, stands upright, thus
turning the middle tooth forwards ; (2) the zygocardiac and pterocardiac
ossicles are straightened out, so that the lateral teeth are brought
together in the middle line. Thus all three teeth meet inside the
proventriculus. The ossicles are brought back to their former position
partly by the elasticity of the walls of the proventriculus and partly by
the contraction of cardiopyloric muscles (Fig. 208).

The filter-chamber is also complicated, having internal
ridges covered with bristles which serve to strain out the

Fig. 209.—- A male crayfish dissected from the dorsal side, after

injection of the arteries.

m.c. , ala cordis ; a.g.m., anterior gastric muscle ; ant. a., antennary artery; c.p.m .,
cardiopyloric muscle; car., cardiac ossicle; d.ab.a., dorsal abdominal artery;
f.c., part of filter chamber, blue coloured in fresh specimens ; d.m., flexor
muscles of abdomen ; g. a., gastric artery; h.g., hind-gut; /it., heart; lr.,
liver; md.m., muscle of mandible; op. a., ophthalmic artery; os., ostium;
fi.g. m. , posterior gastric muscle; prv. provcntriculus (“cardiac” division);
py., pyloric ossicle ; ts. anterior lobe of testis ; tf ., posterior lobe of the same ;
v.d., vas deferens.

302

MANUAL OF ELEMENTARY ZOOLOGY

particles of the food, so that only the finely crushed matter
passes into the mid-gut, while the coarser particles are
passed on into the hind-gut by an apparatus of valves.
Into the mid-gut opens on each side the liver or hepato-
pancreas , a large, lobed, yellow gland, consisting of
numerous short tubes joined by ducts which finally com-
municate with the mid-gut by an opening on each side.
The roof of the mid-gut is prolonged into a short blind
gut or c cecum. Food is either raked up by the third
maxillipeds or seized by the chelipeds and torn up by
them and the smaller pincers. It is passed forwards by
the jaws to the mouth, where pieces are cut from it by
the mandibles and thrust by the mandibular palps and
the maxillules into the mouth. It is chewed in the
proventriculus, strained, and in a finely divided state
passed into the mid-gut. The juice secreted by the liver
digests all classes of food-stuffs, and digestion and
absorption take place within the liver as well as in the
mid-gut. The cuticle of the gut is shed with that of the
body. Shortly before a moult two flat calcareous bodies,
known as “ crabs ’ eyes ” or gastroliths , are laid down in
the forepart of the proventriculus. They are ground up
before the moult takes place. It is uncertain whether they
consist of matter removed from the armour of the body to
weaken it in preparation for the moult or are a store of
material for the strengthening of the new cuticle. Possibly
they serve both purposes.

The heart is a hollow organ with thick, muscular walls.

It is roughly hexagonal in outline, as seen from
Blood vessels. apove^ anc[ pes in the thorax, above the hind-

gut and immediately below the cardiac region of the
carapace, in a space, known as the pericardial sinus , with
membranous walls, to which the heart is connected by six
fibrous bands called the alee cordis. Three pairs of valved
openings or ostia admit blood from the pericardial sinus
to the heart : one pair is dorsal, another lateral, and the
third ventral. From the front end of the heart arise three
vessels — -a median ophthalmic artery , which runs straight
forwards over the proventriculus to supply the eyes and other
organs of the head, and a pair of antennary arteries , which
start one on each side of the ophthalmic, run forwards and

pcm I ov. / . d. ab. a

THE CRAYFISH

303

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304 MANUAL OF ELEMENTARY ZOOLOGY

outwards, and divide each into two branches, one gastric
and the other to the antennae and green gland. Behind
and below the antennaries arise a pair of hepatic arteries ,
which supply the liver, and from the hinder angle of the
heart there is given off a vessel that at once divides into

*rb., Arthrobranch ; br '., outer layer of branchiostegite ; br"., inner layer of the
same ; eff.br. s., efferent branchial sinus ; en.sk., endophragmal skeleton ; ext.m.,
extensor muscle of abdomen; fl.m., flexor muscles of abdomen; g.c., g i 1 -
chamber; h.g., hind-gut ; ht., heart; lr., liver; os., ostia; ov., ovary; pcm.,
pericardium ; pbr., podobranch ; plb., pleurobranch ; st.s., sternal sinus ; v.n.c.,
ventral nerve cord ; v.th.a., ventral thoracic artery; w.L, walking leg.

Small arrows in the sinuses on the right-hand side show the course of the circulation
of the blood.

a dorsal abdominal artery , which runs backwards above
the intestine and supplies it and the muscles of the
abdomen, and a sternal artery. This passes downwards,
through an opening in the ventral nerve cord, and divides
into a ventral abdominal and a ventral thoracic artery ,

THE CRAYFISH

305

by which the limbs are supplied. Each of the arteries
branches many times, till it finally gives rise to minute
vessels in the; organs it supplies, but there are no capillaries.

It will be seen that in the crayfish, as in the earthworm,
there is a dorsal contractile blood vessel, but that in the
crayfish the contractile organ — the heart is very short and
wide, and is prolonged in front and behind by non-contrac-
tile vessels — the ophthalmic and dorsal abdominal arteries.
We shall find in the cockroach a similar but longer heart.

F in 212. — The forepart of the body of a crayfish, viewed from the right-
hand side, with the legs and the branchiostegite cut away and the
gills displayed.

arb.y Arthrobranchiaj ; ep., epipodite of the first maxilliped ; pbr podobranchiae ;
plb.y pleurobranchia ; scg., scaphognathite.

P rom the organs the blood passes into great sinuses which
surround them. The largest of these is the perivisceral
cavity, but there are also blood spaces in the limbs and
elsewhere. The blood from the limbs and a great part of
that from the perivisceral cavity is gathered up into a
sternal sinus , which lies in a tunnel formed by the endo-
phragmal skeleton and contains the ventral nerve cord
and ventral thoracic artery. From this a series of afferent
branchial sinuses carries the blood to the gills, where it is
oxygenated. From the gills it passes by efferent branchial
sinuses to the pericardial sinus. Part of the blood from
20

MANUAL OF ELEMENTARY ZOOLOGY

306

lam.

around the stomach, however, passes on each side into the
space between the two sides of the fold of carapace which
forms the branchiostegite, and thence to the pericardial sinus,
by a vessel which follows the hinder edge of the branchio-
stegite. It will be noted that the pericardial cavity of the
crayfish is a part of the htemoccele and contains blood, un-
like that of the frog, which is a separate part of the coelom.
A blood-vascular system in which, as in the crayfish, the

blood on leaving the arteries
bathes the organs of the body is
said to be open. One in which,
as in the worm and the frog, it
is carried through the organs in
capillaries which lead direct to
veins is said to be closed. The
blood of the crayfish is a clear
fluid, which contains white cor-
puscles and clots readily — an
obvious advantage to an animal
whose open vascular system
causes it to bleed freely from
any wound. A respiratory pig-
ment known as hcemocyanin ,
which is an organic compound
of copper, is dissolved in it, and
plays the same part as haemo-
globin, taking up oxygen in
the respiratory organs and part-
ing with it to the tissues. In
the oxidised condition it is of
a blue colour and tinges the
blood.

The respiratory apparatus of the crayfish is contained
in the gill-chambers. The gills are branched,
thin-walled structures, standing upon the
coxopodites of the thoracic limbs and the
inner wall of the gill-chamber. In them the blood circu-
lates and exchanges its carbon dioxide for the oxygen
which is dissolved in the water that is kept flowing through
the chamber by the action of the second maxilla. This
limb is held firm by the curved end of its endopodite,

base cp.

Fig. '213. —A podobranch of
the crayfish, seen from be-
hind.

Base ; cp. , coxopodite : gill ; lam. ,
lamina; sb., setobranch or
tuft of coxopoditic set* ; slm.,
:stem.

Respiratory

Organs.

the crayfish

307

which fits into a groove upon the mandible at the base of
the palp, while the exopodite or scaphognathite, flapping
at the rate of sixty strokes a minute, bales water forwards,
out of the gill-chamber and under the opening upon the
antenna ot the green gland, whose excreta it thus sweeps
away with the foul water from the gills. By this action
tresh water is drawn into the chamber between the bases
ol the legs. No doubt the blood in the branchiostegite is
oxygenated through the thin inner wall of that organ.

The gills receive different names according to their position. Those
" hbr are. atta('heh to the epipodites of the limbs are known as
podobranchia ?. Others stand upon the membranes which join the
imbs to the body, and are known as arthrobranchia, and a few stand
upon the inner wall of the gill-chamber (the side wall of the thorax)
a ove the legs, and are known as pleurobranchice. A podobranchia is
found on every’ thoracic limb, except the first pair of maxillipeds, which
nave no gills, and the last pair of legs, which have only a pleuro-
-ranchia Two arthrobranchiae, an anterior and a posterior, are found
U 1 * t^iat ^aS a Podobranchia, except the second maxilliped,

which has only the anterior arthrobranchia. Well-developed pleuro-
branchiae are only found above the legs of the last pair, but in the
same position above each leg of the three preceding pairs there is a
minute process which represents a gill. The following table shows
the position ot the gills : —

Maxilli-

Legs.

peds.

Total. 1

.

I.

II.

III.

I.

II.

III.

iv.

V.

Podobranchia! .
Anterior arthro-

Ep

■

I

I

1

1

1

0

6-f Ep

branchiae . ;

Posterior arthro-

t>

T

1

1

r

1

1

0

61 IT

branchiae

0

O

I

x

0

1

s f

Pleurobranchiae

0

0

0

0

R

R

R

5 J

1 + 3R

Total

Ep

2

3

3

3 + R

3 + R

3 + R

1

1

1 8 + 3 R -f- Ep

i

Ep epipodite without a gill. R = abortive rudiment.

Each arthrobranchia has a tree-like structure, consisting of a trunk or
axis arising from the body by one end, with numerous short branches
ox filaments. The two pleurobranchiae have the same structure. In
the podobranchiae the axis is fused to the epipodite along the greater
part ot its length, so that the filaments appear to arise from the

1 In A. fiuviatilis they have each a vestigial arthrobranchia.

3°8

MANUAL OF ELEMENTAL Y ZOOLOGY

epipodite. The tip of the gill, however, stands; free. The epipodite
itself is a long plate with a wide base , a narrower stem and at the
end a second expansion, the lamina. The stem and lamina are
folded along the length of the epipodite, so that a groove is formed,
into which fits the gill of the limb next behind.

The excretory organs of the crayfish are a pair of organs
known as the green glands , which lie in the head,

Fig. 214. — A diagram of the struc-
ture of the green gland of a cray-
fish. Above, the whole gland is
seen in longitudinal section ;
below, the end sac and cortex
are seen as dissected ..out and
viewed from the surface.

bl., Bladder; cor., cortex ; e.s., end-sac ;

med., medulla ; 0., opening on antenna.

Excretory

Organs.

immediately
behind the
antennae,

SO tner/

upon whose basal joints
they open. Each con-
sists of a glandular
mass and above it a
thin - walled bladder
from which a short
duct leads to the open-
ing. In the centre of
the mass is a small,
brownish sac, known
as the end-sac . This
is a vestige of the
coelom, which other-
wise is in the crayfish
represented only by
the hollow of the gonad.
Partitions project into
it from its wall, and
it communicates by a
small opening with the
rest of the mass, known
as the labyrinth , which

is essentially a winding and much complicated tube leading
from the end-sac to the bladder. This tube is a coelomoduct.
Its first section, which forms the outer part of the gland,
known as the cortex, is greenish in colour and broken
into a meshwork of channels. The rest, the medulla of
the gland, is a whitish, coiled tube, simple for a short
distance and then made spongy by ridges of its wall.
The process of excretion by the green glands appears to
be as follows : in the end-sac, as in the glomeruli of the

THE CRAYFISH

309

frog, there is formed a filtrate from the blood, containing
some of its solids but not its proteins. In the cortex other
solids are probably added. In the medulla salts which
were in the end-sac lost by the blood are taken back. The
resulting fluid is of lower concentration than the blood, so
that the body gets rid of water. We have seen (p. 79)
that a similar process occurs in other freshwater animals.
Certain gland cells found on the gills are possibly also
excretory. The principal nitrogenous excreta are am-
monia and amino compounds

In its general plan the nervous system of the crayfish
resembles that of the earthworm. In the front
System* part °f ^ie head, between the green glands, lies a

supra- oesophageal or cerebral ganglion, or brain ,
which corresponds in position to the supra-pharyngeal
ganglia of the worm. It gives nerves to the eyes, anten-
nules, and antennae, . and from it two long circumcesophageal
commissures pass backwards to join behind the oesophagus
in the suboesophageal ganglion. This gives nerves to the
limbs as far as the second maxillipeds, inclusive, and
immediately behind it lies the first thoracic ganglion, which
supplies the third maxillipeds. In each of the remaining
somites of the thorax lies an indistinctly double ganglion
which supplies by several nerves the limbs and other
organs of its somite. These ganglia are set at some
distance apart and are connected by double commissures,
forming thus a ventral cord. Between the fourth and fifth
ganglia the commissures part widely to allow the sternal
artery to pass between them. In the abdomen the cord is
continued and consists o.f a ganglion in each somite united
to its fellows by longitudinal commissures, which are really
double, but appear at first sight to be single. The last
ganglion supplies the telson as well as its own somite.
The commissures contain no nerve cells. The brain is
more complex than those of annelids and exercises more
control over the rest of the nervous system. Giant fibres
run from cells in it along the whole length of the cord and
enable it to bring about sudden movements which involve
distant parts of the body, such as the backward escape
movement.

3io

MANUAL OF ELEMENTARY ZOOLOGY

n. at
n. at. 2

n. m

n. ch.

oes.

■ v. n

oes.

■•■■th. 1

*t. a.

A transverse commissure immediately behind the oesophagus joins
the two circumoesophageal commissures. It contains fibres which
take this roundabout course between the xiortions of the brain which

supply the antennae, thus
indicating that these limbs
belong to the same series
as those behind the
mouth. That is probably
also true of the anten-
nules, and the fact that
the antennules and an-
tennae are innervated
from the supraoesopha-
geal ganglia must be con-
nected with the position
of the mouth, which, as
a result of cephalisation
(p. 280) to a high degree
is farther back than in
the earthworm, where it
c. lies in front of the first
somite. The alimentary
canal is supplied by two-
visceral nerves. The first
has a three-fold origin,
th. 6 being formed by the junc-
tion of a nerve from the
cerebral ganglion with
two nerves which arise

1 Fig. 215. — A semi-dia-
grammatic view of
central nervous sys-
s tem of a crayfish.

ab.i, at>. 6, The first and sixth
abdominal ganglia ; cer.,
cerebral ganglion ; c.ces.,.
circumoesophageal com-
missure ; l.c.y longi-
tudinal commissures of
ventral cord ; n.ab.l.,
nerves to abdominal
limbs ; n.at. 1, nerve to
antennule ; n.at. 2, nerve
to antenna : n.ch.t nerve
to cheliped ; n.m., nerves
to limbs adjoining the
mouth ; o.n ., optic nerve ;
s.ces. 1 suboesophageal
ganglion ; st.a., sternal
artery; th. 1, th. 6, first and
sixth thoracic ganglia ;
v.n. , nerve to proven-
triculus ; v.n'. , nerve to.-
hind-gut.

v. n .

n. ab. 1.

THE CRAYFISH

30

each from a small ganglion on the course of the circumoesophageali
commissure. The second arises from the last abdominal ganglion.

The eyes of the crayfish are compound , containing a.

number of elements, known as ommatidia ,
sense Organs. eacq Qf which is a small complete eye. The

whole eye is black, owing to the presence of pigment in
some of its cells, and is covered with a colourless portion of
the cuticle known as the cornea , divided into a number of
square facets, each of which corresponds to an ommatidium.

The structure of the orpmatidia is complex : each is an elongated-
body consisting of a number of cells derived from the epidermis with,
refractive bodies secreted by them. Over each ommatidium is a
cuticular lens corresponding to one of the facets. Under the lens are
two lenticular cells by which it is secreted. Below this again is a
group of four cells called vitrellce whose inner borders secrete a re-
fractive crystalline co7ie. Innermost is a group of visual cells known
as the retinula. The central borders of these cells secrete an axial
refractive body known as the rhabdome. The inner ends of the visual
cells are continued into fibres which pass into an optic ganglion in the
eyestalk, and from this arises the optic nerve.

The ommatidia are separated by pigment cells and the
retinuiar cells also contain pigment. The way in which
such eyes give rise to vision has been the subject of various-
theories. It appears that the pigment flows about within
the cells, being in weak light retracted inwards and out-
wards so as to expose the sides of the ommatidia, and in
strong light expanded so as to isolate each of them. When
it is retracted the eye gives a single image ; when it is
expanded each retinula gives a separate image of a small
part of the thing seen, sharper than that given when the
eye acts as a whole, though formed with a greater loss of
light, and thus a mosaic image is made up.

The statocysts are a pair of sacs, situated in the basal
joints of the antennules and provided with nerves. Each
has a cuticular lining beset with hairs, with which the nerve
fibres are in communication. Within it are grains of sand,
which are scattered over the opening of the sac by the
pincers and fall into it. It is probable that the principal
function of the organ is informing the animal of its position
by the movements of the sand grains against the hairs, and
thus enabling it to keep its equilibrium. If the statocysts

A. The left eye removed ; B , a portion of the cornea magnified to show the facets ;
C, a longitudinal section of the eye under low magnification ; D, a single
ommatidium highly magnified.— D after Parker.

Outer refractive body or crystalline cone ; cu./., cuticular facet ; epia., epidermis
(hypodermis) , mus muscles which move the eye; «._/!, nerve fibres; otnm
ommatidia , op.g optic ganglion ; op.n., optic nerve ; p.g. , outer pigment cells ;
pg'., inner pigment cells; re/., retinula cells (the sense cells)— these cells
contain pigment ; rh inner refractive body or rhalxlome ; v/., vitrella? or cells
which secrete the crystalline cone.

3 1 2

THE CRAYFISH

313

be removed, the crayfish loses its sense of position and will
often swim upside down. Experiments upon the prawn,
an animal related to the crayfish, illustrate the function of
the statocysts. A prawn that by moulting had lost the

A, The right antennule, seen from the median side with the basal joint opened and
the flagella cut short ; B, basal joint of the left antennule from above ; C , two
hairs from the statocyst. — C partly after Howes.

en., Inner flagellum ; ex., outer flagellum ; grn., sand grains ; n., nerve of the
statocyst ; n.f., nerve fibres ; a., opening of the statocyst ; s/c., statocyst.

lining of its statocysts with the sand grains was kept in
filtered water and supplied with finely powdered iron in
place of sand. When it had placed some of these in its
statocysts, a magnet was brought near it, and by moving
the magnet the particles of iron were caused to move as

3*4

MANUAL OF ELEMENTARY ZOOLOGY

they would be by a change in the position of the animal.
By this means the prawn was made to alter its position in
correspondence with the movements of the magnet. It was
formerly supposed that the statocysts subserved the sense
of hearing , but though the animals appear to perceive
vibrations, and this may be due to the statocysts, it is
doubtful whether the latter are true organs of hearing. We
have seen that the antennules bear on their outer flagella
bristles which subserve the sense of smell. Various of the
setae, especially those of the antennae, are organs of touch .

Fig. 218. — The reproductive organs of a female crayfish.- —

After Suckow.

od. j Oviduct ; ov., ovaries ; ov'., fused posterior part (median lobe);
vu., female aperture on the second walking leg (E).

The sexes of the crayfish are separate. The generative
organs lie in the thorax, above the gut and
Reproduction. pejow pie pericardium. They have the same

general shape in the two sexes, consisting of three lobes,
two anterior and one posterior, with a pair of ducts, which
start from the junction of the anterior and posterior lobes
and run to openings on walking legs. The ovary is
larger and broader than the testis, and has an internal
cavity into which the eggs are shed. The oviducts are
short, straight, and wide ; they open upon the coxopodites
of the second pair of walking legs. The testes consist of
a number of branching ducts which end in small alveoli,.

THE CRAYFISH

3i5

in which the spermatozoa are formed. The vasa deferentia
are narrow and much coiled ; their first part is very slender
and translucent, the second part, which forms most of
the duct, is wider and glandular, and a short terminal,
region has muscular walls which force out the sperm.
The spermatozoa are discs with stiff, pointed processes-
round the edge. The nucleus is a round capsule and to*
one side of this is a small, oval body. Pairing takes place
in September and October.

The male seizes the female,
throws her upon her back,
and passes sperm through the
tubular limbs of his first ab-
dominal segment on to the
parts in the neighbourhood
of her oviducts, the limbs of
his second abdominal pair aid-
ing the process by working
to and fro on the hollows of
the first. The sperm consists
of a sticky substance, se-
creted by the vasa deferentia,
carrying the spermatozoa,
and forms white masses on the
sterna of the female. The eggs,
which are large and yolky,
are laid in November. The
processes of the spermatozoa
adhere to them, and by a
sudden expansion of the con-
tents of the capsule the rest
of the bodA' is forced into the

j

ovum. Each egg is attached to one of the hairs on the
abdominal limbs by a stalked shell formed of a substance
secreted by certain glands on the sterna, and is thus under
the protection ot the mother during its development. By
the division ot the nucleus of the fertilised ovum a syn-
cytium is formed which does not divide into cells until a.
number of nuclei have arisen. The young are hatched at
the beginning of the next summer. They do not differ
greatly from the adult, but have curved tips to the pincers..

Fig. 219. —The reproductive-
organs of a male crayfish —
After Huxley.

/, Testes ; vd., vas deferens ; vd'., open-
ing of vas deferens on last walking;
leg.

3 1 6

MANUAL OF ELEMENTARY ZOOLOGY

by which they cling for a time to the empty shell or the
abdominal limbs of the mother, and are thus protected from
enemies and kept from being swept away by currents and
so eventually reaching the sea, where they would perish.

Fig. 220. — Spermatozoa ol a crayfish.

A, Whole spermatozoon from above ; B, part, enlarged, in section.
cps ., Capsule ; pr stiff processes.

Arthropoda.

Segmented animals with jointed limbs, a thick cuticle,
an open blood-vascular system, and a nervous
system like that of the crayfish are known as
Arthropoda. To this group belong Crustaceans — water-
fleas, Cyclops (Fig. 269), crayfish, crabs, etc. — Arachnids
(scorpions, spiders, mites, ticks, etc.), Myriapods
(Centipedes and Millipedes), and Insects. Crustaceans
( Crustacea ) have two pairs of antennae and are almost all

ANTHROPODA

3*7

water animals, breathing by gills. Arachnids (. Arachnid a )
are without antennae, have four pairs of legs, and nearly all,
by various devices, breathe air. Insects ( Hexapoda ) have
one pair of antennae and are usually land animals, breathing

Fig. 221 . — -A , Two recently hatched crayfish holding on to one of
the swimmerets of the mother ; B , pincers of the young more
highly magnified.— From Huxley.

Ruptured egg cases ; en., endopodite ; #_r., exopodite ; protopodite.

air by tubes which take it direct to the tissues (p. 330) and
winged. They have three pairs of legs. The Arthropoda
are far more numerous than any other group of animals,
and are of very great importance to man, partly because
some of them serve him as food, but more because they

3 1 8 MANUAL OF ELEMENTAL Y ZOOLOGY

•damage his crops, annoy him as parasites, and in sucking
his blood convey to him the germs of very serious diseases.
In spiders (. Araneida ) the first pair of limbs (chelicerce)
are sharp poison-claws, and the second ( pedi -
Araneida. palpi ) are tactile organs. T hese and the tour
pairs ot legs are borne upon a so-called “ cepha-
lothorax ” or prosoma. The animals breathe by means of
• lung-books , which are. pits of the wall of the under side of
•the abdomen, containing a number of leaflets in which the

Fig. 222. — The Garden Spider
( Epeira diademata). — From
Parker and Haswell.

*Note, from behind forwards : abdomen
and prosoma ; on latter, four pairs
of legs, one pair of pedipalpi, one
pair of chelicerae (barely shown).

Fig. 223. — A diagram of a
vertical, longitudinal section
through a lung-book.

a.s., Air space ; /., anterior end ;
h., hinder end; //., “leaves”
of book in which the blood flows ;
o., opening, on v.s., ventral sur-
face of body.

iblood circulates and is thus exposed to the air over a wide
surface. There are one or two pairs of these near the front
end of the abdomen, and at the hind end lie two or three
pairs of spinnerets , from which are secreted the silken
threads that spiders have the power ot forming. Both
.lung-books and spinnerets are regarded as representing
-abdominal appendages.

Mites and ticks (. Acarina ) have neither lung-books nor
lAcarina spinnerets. Many of them live as parasites
upon animals or plants, or on decaying organic
matter. The chelicerae are often transformed into piercing

ARTHROPODA

3i9

organs to enable the animal to suck the juices of its host.
A system of air-tubes ( trachea ) for purposes of respiration
is often present. There is a larva with three pairs of
legs, followed by a stage known as the “ nymph, ” which
has four pairs of legs but has not yet reached the adult form.
During part of this stage the animal is quiescent. Demodex

Fig. 224. — The Follicle
Mite ( Demodex follicu-
lorum ), in ventral view.
— From Thomson.

Fig. 225.— The Itch Mite ( Sarcoptes
scabiei ), in dorsal view. The two
hinder pairs of legs are hidden
under the body. — From Thomson.

folliculorum is a minute, long-bodied mite which lives in the
grease-secreting or “ sebaceous ” glands of the human face.
It is generally harmless, but appears sometimes to set up
skin disease and is accused of spreading the bacillus of
leprosy. In dogs it is the cause of a kind of mange.
Larvae and adults live in the glands and are transmitted by
contagion. Sarcoptes scabiei causes the “ itch ” in man, and
a related species gives rise to mange in dogs. The adult

320

MANUAL OF ELEMENTARY ZOOLOGY

itch-parasite lives in burrows in the skin and there lays its
eggs. The larvae pass to the surface of the skin, where
thev live for the most part under the scabs which the
burrowing has caused to form, till the last stage, when the
female makes the burrow. The treatment consists in baths
and rubbing with various ointments, which generally
contain flowers of sulphur. Neither Demodex nor Sarcoptes
possesses tracheae or eyes. The ticks, of which Lxodes may

Fig 226. — -The Sheep Tick ( Ixodes ricinus),— After Nuttall.

A, Ventral view of male ; B, dorsal view of the same.

be named as an example, are larger animals with tracheae
and often with eyes. Their larvae generally live on and
among plants, but presently seize hold of passing animals,
generally warm-blooded vertebrates, and proceed to suck
their blood. Sometimes they pass their stages on this host,
sometimes they fall off and seek another for adult life.
The host’s skin is pierced by a rostrum formed by the pedi-
palps, which contains the chelicerae and a mid-ventral
piece, the hypostome , which is barbed, so that the parasite
cannot be pulled off. The female swells greatly by gorging

ARTHROPODA

321

blood. The male, which is smaller, swells less. Fertilisa-
tion takes place on the host, and the female then falls off and
lays her eggs, 'l icks spread the minute parasites which
cause various serious diseases, such as red-water fever in
cattle, heart-water in sheep, and tick fever in man.

21

CHAPTER XVI

THE COCKROACH. INSECTS

Common as they now are, cockroaches have only been
introduced into England comparatively recently.
Cockroaches. The first specimens were brought from the East

bv trading vessels at the beginning of the seventeentn century,
and one hundred and fifty years later Gilbert White could
still speak of the cockroach as “an unusual insect” at Sel-
borne. This species was the Common Cockroach, Periplan -
eta orientalis. More recently another species, P . amencana ,
a native of tropical America, has been introduced and is
spreading rapidly. Both are nocturnal insects which
haunt human dwellings, hiding in corners and crevices
by day. They seek warmth, as is natural in view of their
origin, and devour any kind of food they can find.

In its main lines the anatomy of a cockroach resembles
that of a crayfish. The animal is segmented,
Anatomy of the segments (somites) being unlike and
grouped into three regions known as head,
thorax, and abdomen, but these do not correspond with
the parts similarly named in the crayfish. I here is a thick
chitinous cuticle, not moulted by the adult, and some
somites bear jointed limbs. The thorax bears also two pairs
of wings. At the sides of the head lie a pair of large, un-
stalked, compound eyes. The coelom, of which traces are
found in development, disappears in the adult, but there
is a hsemocoelic perivisceral cavity containing blood.

The head is short and deep. Seen from in front it has a
pear-shaped outline, with the narrow end down-
Head’ wards. Its armour consists of several pieces -

two epicranial plates side by side above, two gence at the
sides below the eyes, a frons and clypeus in front. A labrum

332

INSECTS

323

is hinged on to the clypeus below ; its lining is known as
the epipharynx . The appendages of the head are as
follows : There is one pair of long, slender, unbranched,

many-jointed antennae, corresponding to the antennules
ot the crayfish. The second antennae of the latter are not
represented in the cockroach. The mandibles are stout,
toothed structures without palps, not unlike the basal

324

MANUAL OF ELEMENTARY ZOOLOGY

parts oi those of the crayfish. They are followed by a pair
of maxillae, often called first maxillae. These consist of
0) a protopodite of two joints known as the car do and
stipes, (b) a five-jointed endopodite known as the maxillary
Palp , (c) two lobes— an inner lacinia , and a softer outer

galea — borne on the stipes to the
median side of the palp. Behind
the maxillae lie a pair of append-
ages which are known sometimes
as the second maxillae, though
better as the labium. Their
protopodites are fused so that
they form a single lower lip.
This has three joints, the sub-
mentum , the mentum , and the
prementum , which bears on each
side an endopodite or labial
palp of three joints, set upon a
projection known as the palpiger .
At the end of the prementum
stand four lobes, known collec-
tively as the ligula. On each
Fig. 2 28. — A female of side the inner lobe is known as

The ™!s “tewhat »he ^ssa, the outer as the
compressed so as to paraglossa .

show the membranes The head is joined by a soft

Thorax. neck to the thorax.

This consists of three
segments — the prothorax, meso-
thorax, and metathorax. Each
has a tergum or notum above
and a sternum below, joined to
one another at the sides by-
membrane in which lie small

between the abdominal
terga. The legs have
been removed.

ab.i-ab,io, Abdominal terga ;
at., antenna ; c.an., anal
cerci ; h., head ; th.i, pro-

thoracic tergum ; th. 2,
mesothoracic tergum ; th. 3,
metathoracic tergum ; v.g.,

vestige of fore wing.

sclerites — the pleura — which are really basal podomeres
of the legs. The pronotum is the largest and projects in
front so as to hide the neck. Each sternum bears a pair
of legs. The shape of these legs and the names of their
podomeres are shown in Fig. 227. The third and fourth
joints bear bristles which are used in cleaning the body,
the fifth ( tarsus ) is subdivided and bears under each of its

INSECTS

325

subsegments a pad or plant ula , and the last joint bears
two hooked claws used in climbing, and also has between
the claws a pad known as the arolium. The plantulae
and arolium prevent slipping. The mesothorax and
metathorax bear each a pair of wings jointed to the
anterior corners of the notum. The wings are mem-
branous folds of the skin, in which the epidermis has
practically disappeared and the two layers ot cuticle
have come together. Branched ridges known as “ veins
or nervures strengthen the wings. The veins are hollow
and each contains a
trachea (p. 330) and a
nerve. The first pair of
wings are dark-coloured
and horny and form a
cover for the second,
which, when they are at
rest, are folded length-
wise and laid along the
back. In the female of
P. orientalis the wings
are very small. Wings
are not appendages of
the same kind as the
limbs, but movable ex-
pansions of the terga.

The abdomen consists

A KHj'man Of t dl S Oimt C S ,

Abdomen. . 7

each with a
tergum and a sternum,
joined at the sides by soft cuticle. The hinder somites
are telescoped, so that the eighth and ninth are hidden. 1
The first sternum is rudimentary, and the tenth tergum
projects backwards as a plate with a deep notch in
its hinder edge. A pair of many-jointed, spindle-
shaped cerci anales , which may represent limbs, are
attached under this plate, and below it is the anus,
between two podical plates or paraprocts , which may
represent the sternum of an eleventh somite. In the

1 In the male P. orientalis portions of the eighth and ninth terga
remain uncovered.

scr. sut.

Fig. 229. — The head of a cockroach,
seen from in front.

at., Antenna ; clp., clypeus ; ecr., epicranium ;
eye ; fen., fenestra ; fr., frons ; gen.,
gena ; Ibm., part of the labium ; Ib.p.,
labial palp ; Ibr., labrum ; md., mandible ;
nix., part of the maxilla ; mx.p. . maxillary
palp ; sut., sutures.

326

MANUAL OF ELEMENTARY ZOOLOGY

female the seventh sternum is produced backwards into
a large boat-shaped process, which forms the floor of
a genital pouch , and in the male the ninth sternum bears
a pair of limbs in the form of slender, unjointed styles .

FlG. 230. — The mouth-parts of a cockroach. —
From Imms.

I, Mandibles ; ab.m., ad.m., abductor and adductor muscles ;
2, maxilla; c., cardo ; g., galea; /., lacina ; mx.p., palp;
s., stipes ; 3, labium ; gl., glossa ; lip., palp ; m., mentum ;
pg., paraglossa ; pgr., palpiger ; pm., prementum ; sm.,
submentum ; 4, hypopharynx ; si., left vestigial superlingua.

The genital opening is placed below the anus *and is
surrounded by a complicated set of processes known as
gonapophyses. A pair of stink glands, deterrent to most
enemies, open on the membrane between the fifth and
sixth terga. Somites one to eight are limbless.

INSECTS

327

In walking, the legs are used in two tripods. On one

Locomotion side die ^rst ieS Pu^s and the third pushes
while on the opposite side the second leg acts
as a prop. Meanwhile the other three legs are being moved
forwards to repeat the process. In bight, the hinder
wings do the work. They are beaten in such a way as
both to support the body and to drive it forwards. The

Fig. 231.— The ventral aspect of a male cockroach with the wings
extended. An imaginary median line has been inserted. — From
Thomson.

A., Antenn* ; C., cercus ; Co ., coxa, the breadth of which makes it look, in its
present position, like a ventral plate on the body; E., eye ; F., femur ; P.T.,
prothorax ; 67,, style ; TVi., tarsus ; TV., tibia ; 7V., trochanter ; IV*., first pair
of wings ; IV*., second pair of wings.

fore wings are held at right angles to the body and serve
as those of an aeroplane.

They are moved by two sets of muscles — an indirect set, consisting
of vertical and longitudinal muscles of the thorax which by alternately
lowering and raising the tergum, to which the wings are attached,
lever the wings up and down upon the side plates (pleura) upon which
they rest, and a direct set attached to the base of each wing, which
they can both rotate upon its axis and also extend from the body or
retract. The hind-wings are beaten down arid up, and at each
downstroke the strong front edge (costa) is by muscular action

328

MANUAL OF ELEMENTARY ZOOLOGY

rotated downwards and forwards so that the soinewhat concave
lower surface faces obliquely downwards and backwards. This

process is assisted by the resistance of the
air below bending the thin hinder part of
the wing upward. As a result, during
the beat the wing exerts pressure both
downwards and backwards while a
region of decreased pressure is created
above and in front of it. Thus the insect
is pressed and drawn upwards and for-
wards.

dv.m

dv m.

Fto. 232. — A diagram to
show how the wings
of an insect are

The alimentary canal has long,
ectodermal fore- and
System.tary hind-guts, lined with
cuticle as in the cray-
fish. The fore-gut comprises (i)
the mouth, with a tongue-like
ridge ( hypopharynx ) which bears
on its under surface the duct of
the salivary glands and at its sides
a pair of minute structures, the
super lingua, which mav represent
the paragnatha of the crayfish (p.
298) ; (ii) the narrow gullet, lying
in the neck ; (iii) the swollen crop ;
(iv) the proventriculus or gizzard,
which has muscular walls and

lowered and raised in
flight.

A, The downstroke : the ter-
gum (t) is raised, owing
to being arched fore and
aft by the contraction of
the longitudinal muscles
{Lm.) ; this forces the wing
down, pivoting over a point
on the pleuron (pi.). B, the
upstroke : the tergum is
lowered by contraction of
the dorso-ventral muscles
(do.m.) ; this levers the
wing up.

median duct formed

contains six hard, cuticular teeth
and some pads covered with
bristles which form a strainer. A
pair of diffuse salivary glands lie
on each side of the crop, and
between each pair lies a salivary
bladder or receptacle. The ducts
of the two glands of each side
join ; the ducts of the two sides
then unite to form a median
duct, and this is joined by another
by the union of the ducts of

the receptacles. 'The mid-gut or mesenteron, lined by
soft endoderm, is short and narrow and bears at its
beginning seven or eight club-shaped hepatic caeca. The

INSECTS

329

gizzard projects funnel-wise into the mid-gut. The hind-
gut is coiled and divided into a narrow ileum, a wider colon,
and a wide rectum, which has six internal ridges. At the
beginning of the hind-gut are attached a number of long,
fine Malpighian tubules whose epithelium is excretory.

The food is cut up by the mandibles and maxillae,
moistened with saliva, and pushed by maxillae
Excretion and and labium into the mouth ; it is held up for
a time in the crop, where it is acted upon by
the saliva, which digests only starch, and by the mid-gut
secretion which leaks forward. It is then admitted,

little by little, into the gizzard and there broken up
fine by the teeth and strained by the bristles and passes
into the mid-gut. The juice secreted here digests all
classes of foodstuffs : it is secreted by the break up of
epithelial cells, which are replaced from reserve cells.
The delicate, uncuticulate epithelium is protected from
hard particles not, like that of backboned animals, by the
secretion of mucus (p. in) but by a very delicate chitinous
envelope, the peritrophic me?nbrane , which is secreted by
the epithelium but adheres to it only around the entrance
from the gizzard. This membrane is permeable both to
digestive enzymes and to digested food. It is in the
mid-gut that absorption mainly takes place. The pyloric

330 MANUAL OF ELEMENTARY ZOOLOGY

caeca are mere extensions of the mid-gut and do not
differ from it in function. In the hind-gut water, which
is so precious to land animals, is absorbed both from the
faeces and from the urine excreted by the Malpighian
tubules. Nitrogen is excreted as uric acid. In most
insects some is got rid of by the Malpighian tubules and
some laid up in the fatty body (see p. 332), but the cock-
roach appears not to eliminate nitrogen in the urine.

The respiratory
system

organ$a.t0ry consistsof
branch-
ing tubes or trachece,
of ectodermal origin
with a spirally thick-
ened lining of cuticle,
which arise from ten
pairs of openings or
stigmata at the sides
of the body. There
are two large stig-
mata on each side of
the thorax, one be-
tween prothorax and
mesothorax, one be-
tween mesothorax
and metathorax, and
in each of the first
eight abdominal
somites a stigma is placed on each side between the
tergum and the sternum. Air is pumped in and out of
the larger tracheae by contraction and expansion of the
abdomen, and by diffusion renews the gases in the fine
branches of the tracheal system (tracheoles), which have
no cuticular lining, ramify in the tissues, and end upon
or actually in the cells (Figs. 233, 238, 239). Though the
cockroach obtains all its oxygen in this way, much of the
carbon dioxide it discharges is lost through the skin.

When the insect is at rest the ends of the tracheoles are full of fluid.
When the muscles are active products of their metabolism raise the
osmotic pressure in the tissues and this withdraws the fluid so

Fig. 234.-— A diagram of a cross-section
of the thorax of a cockroach, showing
by arrows the course of the circula-
tion.— From Imms.

d., Dorsal diaphragm (floor of pericardium) ; d.v.,
dorso-ventral muscle ; h., heart ; n,, nerve
cord ; s., septum in leg ; v., ventral dia-

phragm.

INSECTS

33i

that air extends more deeply into the tracheoles and reaches their
cells.

The direct supply of air to the tissues is no doubt the
reason for the simple condition of the blood-

Blcod Vessels. . , / • , c , ,

vascular system, which consists ot a long heart
(Fig. 541), lying along the mid-dorsal line of the abdomen
and thorax, an anterior aorta, and a system of ill-defined

332

MANUAL OF ELEMENTARY ZOOLOGY

sinuses, of which the principal is the perivisceral cavity.
The heart is enclosed in a pericardial space and is divided
into thirteen chambers corresponding to the somites.
Each chamber communicates by a pair of ostia at its sides
with the pericardial space. Blood from outlying parts of
the body flows to the perivisceral cavity, thence into the
pericardial cavity through openings in the floor of the
latter, and so through the ostia into the heart, which,
•contracting from behind forwards, drives it through the
.aorta into the sinus system, by way of the sinuses of
the head. Paired triangular alary muscles , whose outer

ends are attached to the terga,
move the pericardial floor, and
thus cause the flow of blood
from the perivisceral cavity into
the pericardial. Both these cavi-
ties are hgemocoelic (p. 275). They
contain a white tissue known as
the fatty body , of whose cells
some hold reserves of fats, carbo-
hydrate, and proteins, others at
least temporarily retain nitrogen-
ous excreta as uric acid, and
others harbour micro-organisms
(bacteroids) which are probably
in some sort of symbiotic rela-
tionship with the insect. The
blood resembles that of the cray-
fish but, as might be expected
in view of the mode of respiration, contains no respiratory
pigment.

The nervous system is on the same general plan as that
of the crayfish. It comprises a pair of supra-
Nervous oesophageal ganglia, which receive optic and

sense Organs, antennary nerves, a pair ot short, wide cir-
cumoesophageal commissures, a suboesophageal
ganglion, and a double ventral cord with a ganglion in
‘each of the first nine segments behind the head. The
alimentary canal is supplied by a visceral nervous system
which receives nerves from the circumoesophageal com-
missures and the brain. Its principal ganglion lies on

S'tn

Fig. 236. — A section through
a lobe of the fatty body of
a cockroach X 650. —
From Imms.

INSECTS

335

the upper side of the crop. I he sense 01 gans include
the large compound eyes, which resemble those of the
crayfish in structure, the antennae, which are tactile and
olfactory, the maxillae, which are saiel to possess the sense
of taste, the anal cerci, which are tactile and also sensitive

sal. r.

hp. C.'

v.n.c.

Mp.t.

\.sal.g.

.... ov .

o.d.
gen.p .

c. a n.

Fig. 237. — A female cockroach, dissected from above.— Adapted

from Shipley and MacBride.

at. Antenna; c.an., arml cerci; <*r.g., cerebral ganglia; <n colon; col.{ ? ool;
letenal eland; cr„ crop; eye ; ven.p., genital pouch, giz., gizzard , h ,
hp. .. hepatic caca; il., ileum ; /&r., labrum ; .g., mesenteron ; 17 ^ Malpighian

tubes; o ., oviduct; or., ovary; rm., rectum; sal. salivary glands »
salivary receptacle; sp., spermathecag ; v.n., visceial nerves, ■. c
nerve cord.

to sound vibrations, various sensory bristles, and possibly
a pair of oval white patches which are found above the
bases of the antennae and are known as the fenestra.

The sexes are separate. The testes are small, paired
organs, embedded in the fatty body below the fifth and
sixth abdominal terga. In the adult the testes are no longer

334

MANUAL OF ELEMENTARY ZOOLOGY

functional. Two vasa deferentia lead backwards and
downwards from them to the ■seminal vesicles,
Reproduction, which are beset with short finger-like processes
and lie side by side to form the so-called mush-
room-shaped gland. The seminal vesicles join behind to
form a muscular tube, the ductus ejaculatorius , which
opens by a median pore between the ninth and tenth
abdominal sterna. A gland of doubtful function, known
as the conglobate gland , lies below the ductus ejaculatorius
and opens with it. The ovaries are paired organs in the
hinder part ot the abdomen, each consisting of eight
tapering tubes, which show swellings corresponding to ova.

There is a single,
short, wide oviduct
which opens on the
eighth abdominal
sternum. On the ninth
sternum a pair of
branched colleterial
glands pour out by
two openings a secre-
tion which forms the
egg-cases. There is
an unequal pair of
spermathecae, which
open between the
eighth and ninth ab-
dominal sterna and store spermatozoa received from a
male. The eggs are laid in cases, each of which contains
sixteen ova and some spermatozoa from the spermathecae.
The young are like the adults, save for the absence of the
wings till after several moults (p. 296). Thus the meta-
morphosis found in so many insects is practically absent.

The number ot different kinds of insects is enormous,
insects. Probably it is not less than 500,000 and equals

that of all other animals together. All insects
resemble the cockroach in the main features of their
anatomy, but many of them depart widely from it in
detail, the differences affecting principally the mouth-parts,
the wings, and the life-history. The mouth-parts vary in
structure with the way in which they are used. In some

FlG. 238. — A diagram of a transverse
section of an insect. — After Packard.

Femur of leg ; g., gut ; h., heart ; nerve-
cord ; sL, stigma ; tr.y trachea; w. wing.

INSECTS

335

insects, as in beetles, they are not very unlike those of the
cockroach, and are used for biting the food. In others, as
in bugs and many flies, they are adapted for piercing the
bodies of other organisms, animals or plants, and sucking
their juices, the labium forming a tube in which lie the

Fig. 230. — A portion of the tracheal tissue of a cockroach, highly
magnified. Only parts of the tubes are in focus.

cu., Cuticulai lining with spiral thickening ; ««., nuclei of the protoplasmic layer;
'ppm., protoplasmic layer continuous with the epidermis (“ hypodermis ”) of the
surface of the body.

other parts in the form of slender knives . or needles
(Fig. 250). In. the house-fly and insects akin to it the
piercing organs are lacking (Fig. 252). In fleas the labial
palps form a tube in which the mandibles lie as stylets.
In butterflies (Figs. 254, 255) the maxillae are long and
grooved, and placed against one another so as to form a
tube or proboscis through which nectar can be sucked

336 MANUAL OF ELEMENTARY ZOOLOGY

J;.

from flowers, the other mouth-parts being vestigial with
the exception of the labial palps. In bees (Fig. 242) the
mouth-parts are adapted for biting and sucking, the
mandibles being not unlike those of a cockroach, and
the laciniae of the maxillae blade-like, while those of the
labium form a grooved structure, along which liquids can
be drawn up. Wings are occasionally absent. They may
both be gauzy and used in flight, or, as in beetles, the
first pair may be hard and horny and form a case for the

second, or, as in
flies, the first pair
alone may be
used in flight and
the second pair
represented by
two minute sen-
sory organs used
in maintaining
balance and
known as hal-
ter es. In butter-
flies both pairs-
Fig. 240,— A semi- diagrammatic view of the are covered, like
hinder part of the body of a male P. ameri- .1 t ^

catta dissected from the right side to show . .

the generative organs. body, With little

an., Anus; c.an ., anal cerci ; cgl., conglobate gland; Sca|eS> which Can
d.ej., ductus ejaculatoriu^ ; gap., gonapophyses ; be brushed Oif aS
nt.s.g., mushroom-shaped gland; m., rectum ; st. g, j

ninth sternum; stl., style ; t., testis ; v.def., vas a powoer.
deferens ; 6-10, terga. T h e 1 i f e - h i S-

tories of insects

are of three types, (i) In certain wingless insects such
Life-Histories. as *be silverfish and springtails, known as
Apterygota or Ametahola , the young very
closely resemble the adults, and the change from the
one stage to the other consists practically only in the
development of the reproductive system. (2) In many
other insects, known as Heterometabola , the young
resemble the adults in form of body, type of mouth-
parts, and the possession of compound eyes, but differ
in not possessing wings. Such young are known
as nymphs. They may change into the adult gradually

INSECTS

337

in a series of moults like the cockroach, or suddenly at a
final moult like the dragonflies, but they never pass through
a quiescent, “ pupal 5; stage. When a free-living young
animal undergoes an obvious change in becoming adult,
the transformation is known as its metamorphosis. The
adult form of an insect is known as its imago , and the
Heterometabola are said to have an incomplete meta-
morphosis. As we have seen, this metamorphosis may be

Fig. 241.' — A semi-diagrammatic view of the
hinder part of the body of a female P. ameri-
cana dissected from the right side to show
the generative organs.

an., Anus ; c.an., anal circi ; col.g., colleterial gland ; gap., gonapophyses ; od.,
oviduct * cm., ovary ; rm., rectum ; sp., spermathecae ; st. 6-st. 9, Sterna •
5-10, terga.

gradual , but when the nymphs are aquatic it has to be
sudden. (3) In the highest insects, including beetles, ants,
bees and wasps, flies, and butterflies, there is a larva which
differs from the adult in body-form, type of mouth-parts,
and the lack of compound eyes ; and in metamorphosis
the insect passes through an almost motionless stage
known as the pupa. These insects are called Holo-
metabola , and in them the metamorphosis is said to be
complete. The body of the pupa, undergoes a profound
reorganisation. A few of the most important systems of
22

338 MANUAL OF ELEMENTARY ZOOLOGY

organs, such as the reproductive, nervous, and circulatory,
last on, but the others are devoured by a .phagocytic action
(p. 124) of the blood corpuscles and re-formed by the
growth of certain clumps of cells, known as imaginal discs ,

which have retained the
embryonic power of grow-
ing into new organs. The
larvse of insects differ
greatly. They may re-
semble the imago in the
general shape of the
body, as in some beetles,
or they may be cater-
pillar-like and have the
thorax ill-marked, as in
butterflies, sawflies, etc.,
or they may be mere
grubs, as in many flies,
bees, etc.

According to these and
other characters the
principal kinds of insects
may be classified as
follows : —

1. Orthoptera. — -Jaws
adapted for biting, wings
usually unlike. Meta-
morphosis incomplete.
Cockroaches, Grass-
hoppers, etc.

2. Odonata . — -Jaws for
biting. Wings alike,
membranous. Metamor-
phosis without pupa.
Dragonflies.

3. Hemiptera. — Jaws for piercing and sucking. Wings
alike or different ; sometimes absent. Metamorphosis
incomplete. Bugs, Lice, Plantlice, etc. The Bed Bug
{Cimex lectularius ) is without wings, save for vestiges ot
the; first pair, which in other bugs are wing-covers. Its
body is flattened, so that it can hide in crevices, and it

Fro. 242.—The head and mouth-
parts of a bee. — After
Cheshire.

a.,

Antenna ; m. , mandible ; g., labrum
or epipharynx ; mx.p., rudiment
of maxillary palp ; mx. , lamina of
maxilla ; Ip., labial palp ; ligula ;
b.f bouton at end. The paraglossae
lie concealed between the basal
portions of the labial palps and the
ligula, opposite the letters pg.

INSECTS

339

secretes from glands a stinking substance. Its eggs are
laid in batches in its hiding-places and hatch in about ten
days. The young resemble the parents, but have no
vestige of wings. They moult five times, feeding before
each moult, and become adult in about a dozen weeks,
the time depending upon the temperature and amount of
food. Bugs can live for more than a year without food.
They have been accused of transmitting the organisms
which cause various diseases, but the evidence of this is
not convincing. They may best be exterminated by
fumigation with sulphur after sealing the infected building,

Fig. 243. — The Bed Bug, Acanthia { = Cimcx) lectularia. — -
From Murray, after Butler.

or by applying paraffin to walls, etc., in such a way as to
work it into all crevices. Lice differ from the bug in
lacking any vestige of wings and having the segments
of the thorax indistinct. They are smaller, and more
dependent on their host, separated from which they soon
die. Pediculus vestimenti , the Body Louse, and P. capitis ,
the Head Louse, live respectively in the clothing and the
hair of the head. They have a life-cycle of about six weeks
and need constant feeding. They are acquired by contact
of person or clothing, and have been proved to transmit
typhus and other fevers. They may be destroyed by
applying paraffin or turpentine to the body and clothing,
or by scalding the latter. Phthirius inguinalis , the Crab

34°

MANUAL OF ELEMENTARY ZOOLOGY

Louse, frequents the hairs about the pubic region of man,
and is conveyed by personal contact.

The Plantlice, of which the well-known Greenfly {Aphis)
is an example, are not only of much practical importance
as pests in gardens, farms, and orchards, but also ot
considerable zoological interest in more ways than one.
They are animals parasitic upon plants ; they are generally

Fig. 244. — The Body Louse ( Fediculus vcstimenti)
In dorsal view. — From Nuttall.

wingless, but spread and often fertilised by means of winged
individuals ; they are in many of their generations vivi-
parous (p. 553) : and in most they are parthenogenetic(p. 593).
Throughout the summer the females reproduce in a manner
which is both viviparous and parthenogenetic, and, as the
young are all females and all capable of producing offspring
in a few days, the plantlice multiply very rapidly. Sooner
or later, generally towards the autumn, there appears a
generation which contains males as well as females. These

INSECTS

34i

pair, and the female lays a fertilised egg which survives the
winter and gives rise in the spring to a parthenogenetic
female again. Most of the generations are wingless, but

Fig. 245.— The Crab Louse ( Prithirius inguinalis).—
From Sedgwick, after Landois.

St., Stigma ; Tr., trachea.

from time to time there appears a generation of winged
females by which the animal is spread ; usually, at least,

a and 4 winged and wingless parthenogenetic females ;
1 and 3 natural size of the same

this happens when the plant is ceasing to afford a good
supply of food. The males are generally winged. Food is
taken at such a rate that much of it passes through the

342

MANUAL OF ELEMENTAL Y ZOOLOGY

alimentary canal but little altered, and oozes from the
hinder opening as a sugary fluid much relished by other
insects,, especially by ants, which in some cases actually
domesticate the aphides in order to obtain it. This fluid,
known as “ honey dew,” was formerly believed to be
secreted by a pair of remarkable tubes which project from
the filth abdominal segment. The great plenty of food
supports the rapid reproduction by which plantlice with-

Fig. 247.— -The Turnip Flea-beetle ( Haltica nemorum ).

From Theobald.

1, Adult, magnified; 2, true length and wing expanse; 3, adult
feeding on leaf ; 4, egg, natural size ; 5, the same magnified ;
6, 7, tunnel made by larva in leaf ; 8, 9, larva, natural size and
magnified; 10, ti, natural size and magnified view of pupa,
which lies in soil.

This very destructive insect feeds, as larva and adult, on
the leaves of turnips, cabbages, broccoli, and other Cruciferae.
It has many broods in the year, the last hibernating under

stones, etc. Its worst damage is done to seedlings. Paraffin,
Derris powder, and a mixture of soot and lime are remedies.

stand the attacks of their many bird and insect enemies.
They may be destroyed by spraying with various mixtures,
containing such substances as paraffin, quassia, and
nicotine, which are injurious to them in various stages of
their lives.

4. Coleoptera. — Jaws for biting. First pair of wings
form a hard cover for the membranous second pair.
Metamorphosis complete. Beetles.

5. Hymenoptera. — -Jaws for biting and sucking (Fig.

INSECTS

343

242). Four membranous wings. Metamorphosis com-
plete. Many live in communities, the majority of the
members of which are sterile females or “ workers.”
Bees, Wasps, Ants, Saw-flies (with caterpillars on plants,
p. 351), Ichneumon flies, which lay their eggs in the larvae
of other insects, especially caterpillars. The ichneumon
larvae live upon the bodies of their hosts till the latter are
killed, when the parasites pupate. In this way thousands
of harmful insects are destroyed (Fig. 258).

6. Diptera.— Jaws for piercing and sucking. Hind
wings represented by minute structures known as halteres,
having the form of a rod ending in a knob, and sensory in
function. Fore-wings membranous. Metamorphosis

A B C

Flo. 248. —The Honey Bee. — From Shipley and MacBride.

A, Male or drone ; B, female or queen ; C, sterile female or worker.

Mosquitoes
and Gnats.

complete. Gnats and Mosquitoes, Crane-flies, House-
flies and Blow-flies, Tsetse-flies (p. 186 and Fig. 119),
Bot-flies, etc.

The mosquitoes (Fig. 123), and the related gnats ( Culex )
are delicately-built insects with slight bodies
(save when they are distended with food),
slender legs, and narrow wings, which, like the
body, are powdered with scales. Their eyes are very large.
The antennae are feathery, but less so in the female than
in the male. The mouth-parts constitute a piercing and
sucking proboscis. The labium forms a long gutter
which contains the slender mandibles (absent in the
male), maxillae, and tongue ( hypopharynx ), and is closed
above by the labrum. The latter is grooved below so as
to form almost a closed tube, and along it the fluid food

344

MANUAL OF ELEMENTARY ZOOLOGY

is sucked up. A pair of long, tactile maxillary palps stand
at the sides of the proboscis. All the mouth-parts except
the labium and the maxillary palps are sharp-pointed and
pierce the tissues of the organism whose juices are being
sucked. The hypopharynx bears a minute groove down
which flows the secretion of the salivary glands. The

Fig. 249- — A ventral view of the head of a fully grown larva
of the mosquito Anopheles maculipennis . — From Nuttall
and Shipley.

b.f Brush with which food is swept from the surface film of the water ( Culex
larvae, hanging with the head down, collect from a lower stratum) ;
c., antenna ; d., palp of maxilla ; stout hairs which arrange the
brush ; k., teeth of mandible ; m., hooked hairs at edge of maxilla ;
p. a median tuft of hairs ; q., a median structure known as the meta-
stoma ; r., rim of head.

labium ends in a pair of labelled, which guide the piercing
organs, and, as the latter penetrate, the flexible labium is
bowed back. The insects normally feed on the juices of
plants, but take blood when they can get it. The male,
however, does not suck blood. There are several genera-
tions in the course of the year. The eggs — which in the
mosquito are boat-shaped — are laid upon the water, where

INSECTS

345

they remain upon the surface owing to the presence ot
floats, containing air, on their shells. The larva (Fig. 126)

has the three regions of the body distinct, though the
thorax and abdomen bear no appendages. It passes its
life hanging, by palmate (palmdeaf-shaped) hairs on its
abdominal segments, from the surface-film of the water,

346

MANUAL OF ELEMENTARY ZOOLOGY

with its dorsal surface uppermost, breathing by a pair of
dorsal stigmata on the eighth abdominal segment, and
feeding by collecting minute organisms and other organic
particles from the water. This it does by means of a pair
of brushes on the head (Fig. 249), which are combed by the
mandibles and maxillae. The pupa (Fig. 126) has a swollen
cephalothorax and clings to the surface-film by two
trumpet-shaped stigmata just behind the head, and by a
pair of palmate hairs at the beginning of the abdomen, the
rest of the abdomen hanging down into the water. It does
not feed. After a few days its cuticle splits down the back,
and the perfect insect {imago) draws itself out, standing

Fig. 251. A mosquito, sucking blood. — After Nuttall and Shipley.

The curved line under the head is the labium.

upon the pupal cuticle as on a raft until its own soft
cuticle has hardened in the air. The mosquito and the
gnat are both British insects. The principal points of
difference between them are shown in Fig. 126 (see also
P- !95)-

The House- Fly, Musca domestica , a heavily-built insect
The House-fly. Wlt^ short> three-jointed antennae, differs
greatly in build and habits, at all stages of its
life-history, from the gnats and mosquitoes, but is much
less unlike the tsetse-fly. Its proboscis, lacking mandibles
and having only the palps of the maxillae, is unable to
pierce, and is adapted only to sucking. As in the mosquito,
the labrum is grooved underneath, and the groove is closed

INSECTS

347

by the hypopharynx, so as to form a tube. The labium
can be folded up, and has at the end a pair of large labellse.
These bear many fine grooves, the pseudotrachea , which
lead to a central spot to which the tip of the labral tube
can be applied, so that fluid collected by the pseudo-
tracheae can be sucked
up the tube by the

pumping action ot the
pharynx. The fly feeds
largely on fluid matter,
but in the presence
of solid food passes
saliva on to it to dissolve
it. It lays its eggs in
rotting matter, by pre-
ference in stable manure,
for which reason heaps
of such substances should
never be allowed to accu-
mulate near houses. The
larvae hatch in one day,
feed on their surround-
ings, and pupate in a
week. They are soft,
white, and legless, with
twelve somites, tapering
forwards, and a head
that can be withdrawn
under the first somite
and carries a pair ot
sharp hook-like man-
dibles and two minute
antennae. The second and

They are soft,

Fig. 252. — A semi-diagrammatic view
of the extended proboscis ot a
house-fly or blow-fly.

hyp., Hypopharynx ; Ibl., labella ; Iltn.,
labium; Ibr., labrum with epipharynx ;
mx.p.. maxillary palp ; ph., par; ot
pharynx exposed by cutting open the
rostrum, the course of the pharynx is
indicated by dotted lines ; pmp., part o
pumping muscles of the pharynx, simi-
larly exposed ; pst., pseudotracheae; rst.,
rostrum, morphologically part of the
head ; sal.d., salivary duct.

last somites bear each

a pair of spiracles, and the fifth and following somites
have each a spiny pad below. i he pupa case is
formed from the last larval skin. The imago emerges
a fortnight after laying, becomes sexually mature in a
week, and lays its first batch of eggs four days after
mating. It may deposit half a dozen batches of 15®
eggs, and this/ with the shortness of its life-history,

348 MANUAL OF ELEMENTARY ZOOLOGY

enables it, in favourable circumstances, to become
immensely numerous. House-flies are dangerous pests,
because they pick up, from excrement, sputum, etc., the
bacilli of typhoid, diarrhoea, tubercle, and other diseases,
carry them on their legs and in their alimentary canal, and
infect food with them. Blow-flies ( Calliphora ), which lay

Fig. 253. — The life-history of the House Fly ( Musca domestica).—

From Theobald.

«, Mandible of larva with adjacent structures ; b, larva j c, anterior spiracle of
the same ; d, eggs ; e, pupa case ; remnants of spiracle on the same.

their eggs in fresh or decaying flesh, including that of open
wounds, have a similar life-history.

7. Aphaniptera . — Jaws for piercing and sucking. No
wings. Metamorphosis complete. Fleas. The body of
these insects is compressed from side to side (thus in the
opposite direction from that of the bed bug), their eyes are
small or absent, ana their legs very long and strong. They
spend part of their time on the body of the host and part
on the ground, where they lay their eggs. The larvae have

INSECTS

349

long, narrow bodies, without legs but with bristles by which
they pull themselves about actively, they have biting

FlG. 254.— A side view
of the head of a
butterfly.

at., Antenna; eye ; l.p .,
labial palp ; rnx.,
maxilla.

FlG. 255.— The head 01 a tiger motb
{Arctia ca/a), seen from in front
and partly from below, after the
removal of the scales.

dp., Clypeus; eye ; Ip., labial palp; lbr.y
labrum ; md., structure supposed to repre-
sent a mandible; rnx., maxillae; tttx.p.,
maxillary palp.

mouth-parts and feed on any kind of organic matter which
they find in dust, etc. After about twelve days (in the case

FlG. 256, — The Common Flea ( Pulex irritans).

A , Larva ; B, pupa ; C, adult.

of the Human Flea, Pulex irritans) they spin a cocoon and
pupate. Various species infest different warm-blooded

350 MANUAL OF ELEMENTARY ZOOLOGY

animals, but many can live for a time on other than their
proper hosts. The plague is transmitted by rat fleas.

Fig. 257>r— The Heart-and-Dart Moth ( Agrotis exclamationis).~
From Theobald, after Curtis.

i. Imago * larva ; 3, earthen case ; containing 4, pupa.

8. Lepidoptera. — Jaws for sucking, formed by the
maxillae only. Four wings alike and covered with scales,

Fig. 258.-- The Green-veined White Butterfly ( Pieris napi ), and the
ichneumon fly that preys upon it ( Hemiteles mdanarius ).—
From Theobald.

1, Imago ; a, egg ; 3, caterpillar ; 4, chrysalis or pupa ; 5, the ichneumon fly ;

6, natural size of the latter.

as is also the body. Metamorphosis complete. Butter-
flies and Moths. Butterflies are the members of one of a

INSECTS

35i

number of groups into which Lepidoptera may be divided.
I hev have knobbed antennae (rare among moths) no
frenulum (a bristle on the hind wing of most moths, which
links it with the fore wing), a habit of folding the wings
upward over the back and not down at the sides, and day-
light flight. The caterpillars of Lepidoptera have biting
mouth-parts, three pairs of true legs, four, or fewer, pairs
of soft “ prolegs,7’ and a pair of “ claspers 77 at the hind
end of the body. They possess silk glands, which open on
a spinneret situated below the mouth. (The caterpillars
of saw-flies have usually more prolegs than those of
Lepidoptera, have a pair on the second abdominal somite
where there are none in Lepidoptera, and lack the ring of
hooked bristles which the prolegs of lepidopterous larvae
possess.) I he butterfly and moth which are figured as
examples in this chapter are both of interest as being
harmful to crops. The dull-coloured larvae of the Heart-
and-Dart Moth live under the soil by day and come out
at night to feed, doing much damage to a variety of plants
in gardens and in fields. The best remedy against them
is a dressing of soot and lime. The Green-veined White
Butterfly lays its eggs upon cabbages and other Cruciferae,
to which the larvae do injury, though they are responsible
for less damage than the Large and Small Whites {Pier is
brassicce and Pieris rapce ), whose habits are similar. Hand
picking is practically the only remedy.

CHAPTER XVII

THE NEMATODA. PARASITISM

Mematoda.

The size of an animal is no measure of its importance
in the economy of nature. All the elephants
and whales together have far less effect on the
course of events in the world than is achieved by some
half-dozen kinds of Protozoa, or again by a few species of
parasitic worms. We have dealt already with some of
these little organisms. There remains one group of them
which now demands our attention. These are the Nema-
toda. The Arthropoda, to which the last two chapters
have been devoted, are the most active and conspicuous
of invertebrate animals. Utterly unlike them in most
respects, yet strangely resembling them in certain particu-
lars, the Nematodes are a group of worm-like animals,
mostly of small size and always of retiring habits. One of
the largest of them is Ascaris lumbricoides , the Human
Roundworm.1

Ascaris lu?nbricoides lives normally in the small intestine
of man. It is a yellowish-white worm, which
reaches a length of 25 centimetres in the female
and 17 in the male, cylindrical, but tapering towards the
ends, and quite smooth. Along the middle of the back
and of the ventral side run dead white lines, and there is
a brownish line along the middle of each flank. At
the front end is the mouth , guarded by three lips , one
above and one at each side below. The dorsal lip bears
at its base two papilla and the ventro-lateral lips one each.
The edges of the lips are finely toothed. Median on the
under side, about two millimetres from the mouth, is a

1 A still larger species is A. megalocephala of the horse, to which the
following description will apply almost equally well.

352

Ascaris.

THE NEMATODA. PARASITISM

353

so-called “ excretory ” pore. The female bears a median
genital pore at about one-third of the length of the body
from the front end, on the ventral side of a region which is
slightly narrowed. The tail is curved downwards, slightly
in the female and strongly in the male. The anus lies
below, about a couple oi millimetres from the hind end.

CL

Fig. 259. — Ascaris lumbricoides.

A , Male ; B, female.

a., Anterior end ; an., anus ; e., “ excretory ” pore ; g.o., genital
opening ; p., posterior end ; p.s., penial setae.

In the male this opening serves also as a genital pore, and
there project from it a pair of penial setce.

Internally, a spacious perivisceral cavity separates a
straight, simple gut from a simple body-wall. The cavity
is traversed by numerous delicate strands of a remarkable
connective tissue, which is composed of processes of a
23

354

MANUAL OF ELEMENTARY ZOOLOGY

few cells, notably of one very large cell placed on the
dorsal side just behind the nerve ring. Over the gut and
the muscle fibres of the body-wall the strands join a thin
covering layer which lines the cavity. Thus the body
cavity may be regarded as intracellular, and on this account
it is held to be neither coelomic nor hsemocoelic.

The body-wall is made up of three layers : a stout,
smooth, albuminoid cuticle which consists of several
layers and is shed four times, an ectoderm (“ hypodermis ?’)
which is without cell-limits and must therefore be classed
as a syncytium (p. 147), and a single layer of peculiar
muscle fibres. The nuclei of the ectoderm, except at the
hinder end, are collected along the mid-dorsal, mid-ventral,

P

Fig. 260. — Ascaris lumbncotdes . — From Sedgwick, after Leuchart.

a. Hind end ot male ; b, head, from above ; c, head, from below ; d, egg, in shell ;

p, “ excretory ” pore ; Sp, penial setae.

and lateral lines. Along these lines the protoplasm bulges
towards the body cavity. A nerve cord is embedded in the
dorsal and ventral lines, and a canal in each lateral line. The
canals have no internal openings ; they unite in front to
open by the “ excretory ” pore, but there is no proof that they
excrete. The two have but one nucleus, which is very
large, and lies in the wall of the left-hand canal, near its
front end. Thus they may be said to be hollowed in the
body of one immense cell. Two more nuclei lie in the wall
of the median duct to the exterior. Four very large,
branched cells lie upon the lateral lines near the anterior
end of the body and have the power of taking up particles

THE N EM A TOE A. PA PA SITISM

355

from the body cavity. They are known from this function
as the phagocytic cells. The nerve cords are connected by
transverse commissures in the ectoderm, and in front join

scrv

Fig. 261. — A transverse section through the middle of the body
of a female Ascaris liimbricoides .

cu Cuticle; d.l., dorsal line; d.n., dorsal nerve; ect., ectoderm.; egg; end., endo-
derm; exx excretory canal; int.% intestine; lateral line; w,/, muscle

fibre; ov., ovary; p.c perivisceral cavity; ut., uterus; p.l., ventral line;
v.n. , ventral nerve.

a ring a little way behind the mouth. From this ring four
other cords run back at the sides, and six forwards. The
nerve ring is slightly thickened above and rather more
below, and contains some nerve cells. The only other
ganglia are placed at the sides of .the nerve ring and at the

MANUAL OF ELEMENTARY ZOOLOGY

356

hinder end of the ventral cord. A few cells are scattered
among the fibrils of which the cords are composed, but
there is no sign of segmental arrangement in these or any
other organ of the body. The number of the cells which
compose the nervous system is small and remarkably con-
stant, each cell being recognisable in the same position in

Fig. 262. — Muscle fibres of Ascaris. — From Parker
and Has well, after Leuckart.

A, A single fibre: B, several fibres in transverse section,
with a portion of the ectoderm (below).

c, Contractile part of the fibre ; f, processes ; nu, nucleus;
p, undifferentiated protoplasmic part of the fibre.

every individual. Each muscle fibre consists of an outer
longitudinally-fibrillated part and an inner part of un-
differentiated cytoplasm containing a nucleus. Strands of
protoplasm stretch from the inner parts of the muscle fibres
to the dorsal and ventral nerves. The alimentary canal
consists of three parts : a short stomodaeum, or fore-gut,
known as the pharynx, a long mid-gut, and a short procto-
daeum, or hind-gut, known as the rectum. The fore- and

THE NEM A TOD A. PARASITISM

357

hind-guts are lined by inturned ectoderm, with a prolonga-
tion of the outer cuticle, which is shed with the latter.

They have in their walls muscular
fibres. The mid-gut is composed
solely of a layer of columnar epithe-
lium, with a basement membrane
outside it. The food consists of
solids and liquids taken up from the
contents of the intestine of the host.

There is no vascular or respiratory
system. Excretion probably takes
place through the cuticle.

The genital organs are of a type
peculiar to the Nematoda. They are
paired in the female and unpaired in
the male, and lie free in the body
cavity. The male apparatus is com-
posed of (a) the testis, a long, coiled
thread consisting in its anterior part
of a solid mass of immature sex-ceils,
and in its hinder part containing a
cord or “ rachis in the middle with
riper sex-cells attached to it, (p) the
vas deferens of much the same width
as the testis, (V)the vesicula seminalis,
a wider tube, ( d ) a short, narrow,
muscular ductus ejaculatomus. The
spermatozoa are simple rounded cells,
Fig. 263 - A diagram of which become amoeboid when they
the nervous system have been transferred to the female,
of a nematode.— female organs correspond with

rfte0rmBufschf.W1C ’ those ot' the male: Each 0Va^ haS

the same general structure as the
testis, a hollow region which may be

A, Anus ; Ag, anal ganglion ;

Bn, ventral nerve; C , lv,ou>3, u, ,, ... j

LSflT °nrint • called the oviduct connects it with the

cumoesophageal ring
Rn, dorsal nerve ; A",

lateral nerve ; SI, sub-
lateral nerve; Sm, sub*
median nerve.

wide uterus, and the two uteri unite
in a short, muscular vagina. The eggs
are produced in immense numbers
— some 1500 a day— fertilised in the upper part of the
uterus, enclosed in a chitinoid shell, passed into the gut of
the host, and by it voided with the faeces. Before they can

35*

MANUAL OL ELEMENTARY ZOOLOGY

bring about a new infggtion, they must pass through a
period of ripening, whichineeds moisture, a temperature
above 6o° F., abundant oxygen, and the absence of putre-
faction, and therefore cannot take place except in the outer
world. The most ^favourable situation for it is the upper
layer of damp soil. The eggs stand drought, and, by being
swallowed with food or water, , may reach a new host.
Usually they hatch in his intestine, but in warm, damp
places may do so in soil. In the egg occur the first two of
the four moults which Ascaris, like other nematodes, under-
goes in the course of its life. The worms hatch as infective
larvce which in the new host do not at once become intes-
tinal parasites but undertake first a remarkable journey.
Freeing themselves from the remains of the second cuticle
and piercing the wall of the intestine, they enter venules
and lymphatics and are carried through the heart to the
lungs, where they cause congestion and haemorrhage and
are thus discharged into the alveoli. Thence they travel
along the bronchi and trachea into the gullet and descend
the alimentary canal to reach the intestine once more. On
this journey they undergo their remaining two moults,
acquire as swimming-organs lateral membranes, which they
afterwards lose, and grow from .28 mm. to 2 mm. in length.

Ascaris lumbricoidcs is found throughout the world,
and can only be avoided by care taken in regard to the
cleanness of raw foods and drinking water. It may cause
little trouble to the host, or be the source of diarrhoea,
and of anaemia and other complaints, the latter apparently
through an enzyme formed by it which interferes with
digestion. In severe infections with the eggs, temporary
bronchitis may occur during the passage of the larvae
through the lungs. Santonin, thymol, and other vermi-
fuges are used against the worm.

Though many nematode worms are known, none of them
has been found to differ anatomically from Ascaris in
any important respect : all are either, like it, parasitic, or
live in damp spots where decaying matter is plentiful ;
and the same simple organisation is adapted by slight
differences to the needs of each of them. Their life-histories
however, are as diverse as they are remarkable, probably
because it is only by strange and various shifts that they

Fig. .264.— A male Ascaris turn-
bricoides dissected from above.

d.e., Ductus ejaculatorius ; p.s., sacs
of the penial *etae ; 1 ., testis ^ v.d.,
va* deferens ; r.x., vesicula seminalis.

Fig. 265. — A female A scar is turn-
bricoidts dissected from above.

ini., Intestine ; lateral lines ; od ., ovi-
duct; ov., ovary; ph., pharynx; rm.,
rectum ; ut.% uterus; vJ., ventral line

359

MANUAL OF ELEMENTARY ZOOLOGY

3°°

can obtain entry to their several hosts. The following are
brief outlines of examples of the principal types ol nema-
tode life-history : —

1. Free-living throughout life. — The Vinegar Worm, Anguillula

aceti , found in vinegar. Rhabditis, in soil.

2. Free as larvae, parasitic m plants as adults. — The Cockle Worm,
Tylenchus scandens , cause of “ ear-cockles ” in corn. Small worms
from the soil wriggle up the stems of young corn plants, pair in the
flowers when these form, cause galls to arise in place of grains, and
lay eggs, from which hatch larvae. These can survive dry for twenty
years but in damp earth become active.

Fig. 26t>. — The Vinegar Worm (. Anguillula aceti), somewhat dia-
grammatic, to show arrangement of organs.

A, Male ; B, female.

an., Anus ; e., excretory pore ; g.o., genital opening ; int., intestine ; oes.b., bulb of
oesophagus ; ov., ovary ; p.s., penial setae ; ph., pharynx ; t., testis ; nt., uterus ;
v.d., vas deferens.

3. Free as larvae, parasitic in animals as adults. — The Miners’ Worm,
Ancylostoma duodenale, of a pink colour, the male 8-1 1 mm. long,
the female io-i8mm, lives and pairs in the small intestine of man.
Eggs are passed with the faeces of the host and hatch in warm, damp
places, but are killed by drought or frost. The little, thread-like larvae
pierce the human skin, usually on the foot, hand, or mouth, enter
venules or lymphatics, and are carried in the blood through the heart to
the lung wall, which they penetrate. Thence they reach the gut by way
of the glottis. Browsing on the villi, they cause intestinal haemorrhage,
and thus anaemia, often fatal. The worm is widespread in warm countries,
but elsewhere can exist only where the conditions are favourable, as in
mines in Britain. Strict sanitation is necessary for avoiding its ravages.
Male fern and thymol are the principal vermifuges used against it.

THE NEMATODA. PARASITISM

361

4. Larva parasitic , adults free. — The Rain Worm, Mermis nigres-
cens, whose larvae bore through the skin of young grasshoppers and
live in the body cavity till they are full grown, when they escape into
■damp earth, become sexually mature, and pair. After rain the adultsi

Fig. 267. — The Corn-cockle Worm. — From Theobald.

A, Cockle gall ; C, larvae ; in D, gall cut open ; E, larv® magnified.

sometimes climb the stems of plants in such numbers as to give rise
to the legend of “ showers of worms.”

5. Larva and adults parasitic in different animals, with a free stage.
—The Guinea Worm, Dracunculus medinensis. The female, about

Fig. 268. — The Miner's Worm ( Ancylostoma duodenale). — From
Parker and Halswell, after Leuckart.

A, Male and female in coitu ; B, anterior end ; C, mouth, with spines ;

D, hinder end of male, with expansion known as bursa,
cv.g., Cervical glands ; ph., pharynx.

•90 cm. long, encysts beneath the skin of man, usually in the leg, with
the head in the host’s foot, causing an abscess. She is viviparous.
The young escape through the abscess when the host wades in water,
and enter the small crustacean Cyclops , with which they are swallowed
by a new host. The male is small and has rarely been seen. The
disease is common in tropical Africa and parts of Asia. The worm is
removed by coiling it on a twig. If it be broken during this process.

362 MANUAL OF ELEMENTARY ZOOLOGY

and larvm escape into the host’s tissues, sepsis, fever, and even death
may result.

6. Larva: and adult parasitic , without free stage, in animals of
unlike kinds. — The supposed cause of elephantiasis, Filaria bannofti ,

ats

Fig. 269. — The Guinea
Worm ( Dracunculus
medinensis).

A , Adult female, reduced;
B , larva, much magnified.

Fig. 270. — Cyclops.

ab .1, F'irst abdominal segment ; atd, antennule ;
aid, antenna ; c.f., caudal fork ; cph.t
cephalothorax (fused head and first two
thoracic segments); e.s., egg sac; eye
(single and median); g alimentary canal ;
gen.op., genital opening; t., telson; thd,
thd, third and sixth thoracic segments.

In comparing this crustacean with the
crayfish, note the absence of proventriculus,
paired eyes, uropods, and carapace, the
presence of median eye and caudal fork,
and the difference in the number of seg-
ments.

lives and pairs (male 8-9 cm., female 15 cm.) in the human lym-
phatics. The femalg is viviparous, setting free embryos into the lymph,
whence they reach the blood and are sucked up by mosquitoes or gnats,
in which they bore through the wall ; and develop in the muscles,
loiter they enter the salivary glands and are injected into man during

THE NE MATO DA, PARASITISM

363

blood-sucking. They may cause no ill effects, but the adults may block
lymphatics, or unripe eggs, freed by injury to the mother, being
broader than the larvae, may choke capillaries. Thus dropsy, and it
is believed elephantiasis, are caused. Three kinds of Filaria have
similar habits : F. Persians, whose larvae remain in the surface vessels
of the host day and night ; F. bancrofti (or F. nocturna), whose larvae
retreat to deeper vessels, out of reach of mosquitoes, by day ; and

Fig. 271. — The
female of Filaria
bancrofti.

FlG. 272. — Filaria bancrofti . — From
the Transactions of the Society of
Tropical Medicine, after Leiper

A, Head ; B, tail of male— showing the arrange-
ment of papillae characteristic of the species.

F. loa (or F. diurna), whose larvae retreat at night. The carrier of
F. loa is the blood-sucking fly Chrysops. If the patient be made to
sleep by day, the larvae reverse their habit. The parasites cannot be
destroyed, but may be avoided by measures against their insect-
carriers. They are widespread in warm countries.

7. Adult and larva parasitic , without free stage , in the same host. — •
The cause of trichinosis, Trichinella spiralis , lives, when it is adult,
in the small intestine of rat, pig, or man, and there pairs (male 1 mm.
long, female 3-4 mm.). The females penetrate to the intestinal
lymphatics, and produce viviparously young which reach blood
vessels, and are carried to muscles, where they encyst. The tissues
of the host enclose the cyst in a capsule, and the larvae remain till
the host be eaten by another individual of the same or another
species, when they become adult in the intestine. The symptoms
produced in the host are intestinal catarrh during the piercing of
the gut wall, rheumatic pains and fever during migration, and
general cachexia during encystment. The disease may or may not
prove 'fatal. It can be avoided by meat inspection and thorough
cooking. It is found in all parts of the world.

8. No larval stage. Adult parasitic dn the gut of a vertebrate : eggs
pass out in faces and are swallowed by another individual of the same

364 MANUAL OF ELEMENTARY ZOOLOGY

■host. Trichocephalus or Trichuris (male 30-45 mm., female 35-
30 mm.) lives in the human caecum, appendix, and colon, attached

Fig. 273. — Trichinella spiralis , young encysted in muscle of host.—
From Thomson, after Leuckart.

Fig. 274. — Trichocephalus dispar. — From Sedgwick, after Leuckart,

Kgg , b, female J c, male, with forepart of body buried in mucous membrane

of host ; S/>, penial seta.

THE NEMATODA. PARASITISM

by its long whip-like fore-end. It is generally harmless, but may cause
inflammation. It is widely distributed. Oxyuns (male 2—5 mm.,
female 9-12 mm.) has the same habitat as '1 rtchocephalus. It rarely
causes more than irritation. The eggs
are ripe when they leave the host’s
rectum, so that self-infection by him is
possible. The animal is cosmopolitan.

As a precaution against infection with
these parasites and others of similar life-
history, the thorough washing of raw
vegetables and strawberries is desirable.

9. A free bisexual generation alternates
with a parasitic hermaphrodite.— Rhab-
donema nigrovenosum. The herma-
phrodite is a blackish worm which lives
in the lungs of the frog, where several
specimens may often be found in dis-
secting. It is protandrous. Embryos
escape by the glottis, are passed with
faeces, and become sexual adults. Their
young wander, and are swallowed with
food by a frog. An example of hetero-
gamy (pp. 248, 249).

The foregoing paragraphs give
only a small sample
Parasitism. ^ ^ ^ . histories

of Nematoda. These animals
have experimented with almost
every kind of host, and almost
every kind of internal parasitism,
and in every line their organisa-
tion betrays their adaptation to
this way of life. As they are the Fig 275 —Oxyuris, some-

' - • • \xr hot n tonro m no o 1 1 r* to*

last important group or parasitic
animals with which we have to
deal, we may pause here briefly

to survey the phenomena o para Anus; g.o.t genital opening;
sitism. A parasitic organism is *'»/., intestine; oes.b., bulb

one which lives on or in the body
of another without conferring any
advantage on it. Parasites may be
external, as the Greenfly (Aphis)
or the Flea, or internal, as those with which we have just
dealt. An internal parasite may live (i) in the hollows of
the body of its host, as Entamoeba , Tania, and Ascaris in the

what diagrammatic, to
show arrangement 0$
organs.

A, Male ; B , female.

f WWW — .,

of oesophagus ; ov., ovary ;
e.h., pharynx ; testis ; ut.
uterus ; v.d ., vas deferens
vag., vagina

A 4A

366

MANUAL OF ELEMENTARY ZOOLOGY

gut, and Trypanosoma and Filaria in the blood vessels and
lymph spaces, (ii) in the tissues, as Dracunculus and young
Trichinella, or (iii) in protoplasm, as the young trophozoite
of Monocystis , or that of the malaria parasite. Its life is
passed in the following conditions : (i) Food is plentiful,
needs no hunting, and is usually easy of digestion, (ii) there
are no enemies to be escaped, (iii) means of fixation (e.g to
the gut-wall) are sometimes needed, (iv) provision must be
made for distribution from one host to the next, (v) the
surroundings are often devoid of free oxygen. We find
accordingly in internal parasites (i) an absence or poor
development of organs of locomotion, of defence and
offence, and of sense, (ii) simplicity in the organs of nutrition,
(iii) often the presence of organs of fixation (suckers, hooks,
etc.), (iv) many adaptations of the life-history to distribution.
Such are : (a) the vast numbers in which the young are
usually produced, so that some survive the “ passage
perilous,” ( h ) the occurrence of special distributive indi-
viduals, either active, as the miracidium of the liver fluke
and the larval stages of Tylenchus, or adapted to passive trans-
mission, as the gamonts of the malaria parasite, various
encysted Protozoa, and hard-shelled eggs of worms — thus the
parasite has a multiplicative phase in which it takes advan-
tage of the rich supply of food in its host to produce many
offspring, and a distributive phase in which temporarily
non-reproductive individuals find new hosts — ( c ) often the
passage through an intermediate host. When there are
two different hosts, the principal host is that in which the
parasite passes its sexual phase (not necessarily the larger
host, as is seen in the case of the malaria parasite). This host
usually preys on the intermediate host.1 The intermediate
host of the malaria parasite is man, that of the pork tape-
worm is the pig, that of Filaria is the mosquito, that of the
guinea worm is the freshwater crustacean Cyclops. The
advantages of this arrangement are twofold : (i) the parasite
finds a means of re-entering its principal host, (2) very often
the intermediate host provides a richer supply of nourish-
ment than the principal host, and the parasite there under-
goes most of its growth and reproduces asexually. (v) In-
ternal parasites often obtain the energy for their life,
1 Filaria bancrofti is an exception to this.

THE NEMATODA. PARASITISM 367

by an anaerobic process, the complex molecules of
carbohydrates being decomposed to form simpler ones
without the intervention of free oxygen (see p. 4).

To this end such parasites lay up in their tissues large quantities
of the starch-like substance glycogen of which carbohydrate stores
in animals are usually composed (p. 62). Thirty per cent, of the
dry weight of an A sc cltzs , and nearly half that of a tapeworm, con-
sist of this substance. The glycogen is decomposed according to the
following equation :

(C6H10O6)»+»H2O - 2«C3H603

(Glycogen) (Lactic acid)

As we have seen, this process yields considerably less energy than would
be obtained by total oxidation. Apparently, at least in many animals,
it cannot go on indefinitely unless the lactic acid be removed. This
may happen either by the acid being discharged into the surrounding
fluid and swept away by movements of the latter (which would occur,
for instance, in the host’s intestine), or by an access of oxygen with
which some of the lactic acid is oxidised so as to give energy tor
building the rest back into glycogen. In the latter case the process
becomes ultimately aerobic. It is probable, indeed, that even in
aerobic animals the process by which energy is liberated is at first
anaerobic, but that this phase is quickly followed by one in which,
by the use of oxygen, a part of the product is destroyed and the rest
built back, so that the process as a whole appears aerobic. _ Ihus
anaerobic animals differ from those that are aerobic only in the
length of time for which the anaerobic process goes on.

It should be noted that parasites are not necessarily
harmful. In fact, those which are so have not yet reached
that accommodation with their host which the course of
generations usually brings about. An organism which
suffers injury from a parasite, either by loss of food, or by
destruction of tissues, or by poisonous by-products, fights
the intruder in various ways — by expelling it, by attacking
it with leucocytes, and by the secretion of counter-poisons.
It also lessens the effects of the parasite by taking enough
food to supply it as well as itself, and, if poisons be in
question by the secretion of “ antitoxins ” which neutralise
them It is usually to the interest of the parasite not entirely
to overcome these measures, for by so doing it deprives
itself of a host. Most parasites have reached an equi-
librium with their host, which is able to keep them from
destroying, and often even from harming it, but cannot

destroy them altogether.

368

MANUAL OF ELEMENTARY ZOOLOGY

Allusion has already been made to certain broad resem-
blances between the Nematoda and the Arthro-
Artn?o»odaa,lti poda. These may now be specified. The stout
cuticle, shed at intervals, with its underlying
syncytium, both inturned in the fore and hind guts, the
lack of perivisceral coelom, the absence of cilia, and the
unflagellated spermatozoa have been held to show an
affinity between the groups. But the excretory system,
much of the histology, and some features of the nervous
system of the Nematoda are entirely peculiar to that
group, and both segmentation and limbs are wanting in
them. The Nematoda, indeed, are one of the most isolated
groups of the animal kingdom.

CHAPTER XVIII

THE SWAN MUSSEL. MOLLUSCA

Freshwater mussels may be found in most streams.
Habit* canals, and large ponds in Britain, though they

are often overlooked on account of their habit
of burying themselves in the mud with at most a small part
of the body projecting. The commonest of them is the
Swan Mussel, Anodonla cygnea. When it is removed from
the mud it is seen to be enclosed in a flat, dark-green-
shell, four to six inches long and roughly oval in outline,
with one end (the front) rounded and the other more
pointed. The shell consists of two similar pieces, known
as valves, which lie one on each side of the animal
joined by a hinge above the back, where their edges are
almost straight. On being disturbed the mussel holds the
valves tightly together, but when it is at rest in the water
they gape somewhat, and at the hind end, which projects
slightly from the mud, there may be seen between them
two fleshy lobes enclosing an opening shaped like a figure
of 8, through one of whose limbs a current sets into the
shell, while through the other, the upper of the two, the
water is driven out. At times the animal moves about,
thrusting out a yellowish, ploughshare-shaped organ known
as the foot , with which it ploughs its way through the mud
at the rate of about a mile a year. Freshwater mussels are
not unfit for food and are sometimes eaten. They are preyed
upon by water-fowl and other animals, and in places are
fished for on account of the pearls which they contain,
which may be of considerable value. They are not killed
by the freezing of the water even if they themselves be
frozen solid, but can only survive a few hours of drought.

The shell consists of an outer horny layer, th t periostracum*
24

370

MANUAL OF ELEMENTARY ZOOLOGY

Shell and
Mantle.

a thick middle prismatic layer impregnated with salts of
lime, and an inner nacreous layer of mother-
of-pearl, which consists of ’thin calcareous
laminae. Lines of growth parallel with its edge
mark the outside of the shell, centering upon a point about
a quarter of its length from the front end. This point is
known as the umbo and shows the position of the first shell

i.a.cul.

i.pro,

Pal

A

FiCi. 276. — The Swan Mussel.

A, The shell with the animal, from the right side ; B, the left valve of the shell, from
within.

d.s., Dorsal siphon; /., foot; /., impressions of muscles; i.a.ad., of anterior
adductor; i.a.r., of anterior retractor; i.p.ad., of posterior adductor •pi.p.r.,
of posterior retractor; i.pro., of protractor; l.g., lines of growth; pal., pallia!
line ; ub., umbo ; v.s., ventral siphon.

THE SWAN MUSSEL

37i

of the young mussel. On the inside of the shell may be
seen the marks of at tachment of the adductor, retractor,
and protractor muscles presently to be mentioned, and
parallel with its edge is a mark known as the pallial line ,
where the fold M the body-wall known as the mantle is
attached: Abbve the hinge the two valves are joined by an
elastic ligaments^ which pulls them together and thus causes
them to gape below when the adductor muscles are relaxed.
To open the shell of a living mussel the blade of a knife is
passed between the valves and they are prised apart till the

muscles can be cut close
to the shell on one side.
The body of the animal is
then found to be soft, with-
out a cuticle, and provided
with a flap of tissue which
hangs down on each side
and covers the other organs.
This is the mantle. It has
a thick edge which secretes
the two outer layers of the
shell, while the pearly layer
is laid down by the whole
outer surface of the mantle
and skin of the back.
Pearls are formed in the
same way in pockets of
the mantle surface around
foreign bodies which have
intruded between mantle and shell. The origin of the
mantle from the side of the body is not straight but
higher in the middle than near the twro ends, though at the
extreme ends it turns upwards to the hinge line. At the
hind end each mantle edge is fused for some distance with
its fellow ; it then separates widely from it twice, so as to
form the figure of 8 already mentioned, and lies against
its fellow for the rest of its length. The upper opening is
known as the dorsal siphon , the lower as the ventral siphon .
The lips of 1 the latter bear a fringe of small tentacles.
The space enclosed by the two mantle lobes is known a.s
the mantle cavity .

swan mussel, seen from above.

Lines of growth ; lig., ligament ;
ub umbo.

372

MANUAL OF ELEMENTARY ZOOLOGY

It will have been noticed that the shel! and mantle of
_ ^ . the mussel are bilaterally symmetrical. The

Feature*: same symmetry is found in all the other organs

amMtoetUnff k°dy> b01;h internal and external. Above

the attachment of the mantle, at its lowest point
near each end, may be seen on each side the cut surface
of the great adductor muscles , anterior and posterior, which

Fig. 278. — A swan mussel removed from its shell and lying
on its right side with the greater part of the left lobe of
the mantle cut away.

Anterior adductor muscle; a.r., anterior retractor; d.s., dorsal
siphon ; f , foot; left inner gill ; LmL, remains of left mantle

lobe turned back; l.o.g., left outer gill; labial palps; p.ad

posterior adductor muscle posterior retractor ; pro., protractor ;

r.ml., right mantle lobe; r.tnt., thickened edge of the same; v.s.,
ventral siphon with papillm.

pass through the body from side to side and draw together
the valves of the shell. To the upper and inner sides
of these lie the anterior and posterior retractor muscles v
which draw the body forwards upon the foot when the
latter has been thrust out. Behind the lower end of
each anterior adductor is a protractor muscle, which draws
the body backward upon the foot. On turning back
the mantle the rest of the external organs are laid bare.
The most conspicuous of these are the foot and two pairs

THE SWAN MUSSEL

373

of flaps which hang down on each side of the body
One pair is large, extends from the hind end along the
greater part of the length of the animal, and consists of
the gills. The other is smaller, lies in front of the gills, and

is known as the labial palps. The loot is a wedge-like
structure with an angle directed forwards, placed under
the front half of the body. Its lower part is muscular, its

the openings of the kidney and gonad.

■a. ad., Anterior adductor muscle; ci.s., dorsal, siphon ; toot ; gen. , opening of the
duct of the gonad ; k.o., opening of the kidney ; l.i.g., left inner. gill l.ntl. , Jett
mantle lobe ; /.<?.£■., left outer gill \ AA* labial palps ; r.i.g'.y right inner gib *
r.ml., right mantle lobe; r.mH ., thickened edge of the same; r.o.g ., right
outer gill ; z'.s., ventral siphon.

upper part soft, containing the genital organs and intestine.
It is thrust out between the valves by the forcing of blood
into the sinuses which it contains, and withdrawn by the
removal of the blood by the contraction ot muscles. In
locomotion it is wedged into the mud or between stones,
and the body is then drawn forwards upon it by the
retractor muscles. Above the foot, between it and the
anterior adductor muscle, lies the mouth, bordered by
upper and lower lips. At the sides of the mouth these lips

374

MANUAL OF ELEMENTARY ZOOLOGY

are continuous with the labial palps , which are a pair of
triangular flaps, one outside the other, on each side of ihe
body, the outer palps being joined in front of the mouth
by the upper hp and the inner behind the mouth by the
lower hp. The palps are ciliated, and their surfaces which
are towards one another are crossed by fine furrows,
whose cilia work outwards. The groove between the palps
leads to the corners of the mouth, and the whole forms an
apparatus by which small organisms and other fine organic
particles which are gathered from the water by the gills

FfG. 2S0. A, A horizontal section through a gill of the twnn

SSSSSEf-” ; • “>'• «4t*»K5KS5

W2 ; *’«•!»»» ; «de

interlamellar junction • i l s-b interlamella^'c lnterfilameiJtar junction ; i.l.j.,
laterofrontal cilia; 0 outer side of fiSnt • ^ ! ^ lateral cilia ’ *•/•'•*
skeletal rods which support each filament. ’ k’r'* Sectlons of the chitmous

are sorted and swallowed or rejected. Each of the rills
consists of two lamella continuous along their ventral
edges. As there are two gills on each side of the body

comnotod °f eaCh Slde f°Ur Iamell9e- Each lamella is
3w i °l Very nTerous vertical filaments which in-
f ^ (*at ^ at the Slde towards the other lamella)
K,f^sec) together at irregular intervals, so as to form a
ribbed plate pierced by numerous openings between ihe
ribs leading into the interlamellar space of the gill The
filaments of the two lamellae of a gill are continuous, ea< h

THE SWAN MUSSEL

375

filament passing down one lamella and up the other, so
that the whole gill may be said to be composed of a
number of bent filaments fused side by side so as to form
two lamellae. The two lamellae of each gill are connected
at intervals by thick vertical folds parallel to the filaments
The lamellae diverge upwards, so that in transverse section
the two gills of each side have the form of a W. The
space into which each interlamellar space widens at the top
is known as an epibranchial space. The outer lamella of the
outer gill of each side is attached along the whole length of

Fig. 281. — Diagrams of transverse sections through the swan mussel.

A passes through the middle of the foot and shows the inner lamella of the inner
gill attached to the side of the foot ; B passes through the hinder part of the
foot and shows the inner lamella of the inner gill free ; C is taken behind the
foot and shows the inner lamella? of the inner gills joining in the middle line ;
D is further back and shows the axes of the gills free.
ax.g., Axes of the gills ; cl.c., cloacal chamber ; ep.sp., epibranchial space ; /., foot ;
i.g. 1, inner lamella of inner gill ; i.g. 2, outer lamella of inner gill ; il.sp., inter-
lamellar space ; o.g.i, inner lamellae of outer gill ; o.g. 2, outer lamella of outer
gill ; ml., mantle lobe ; mix., mantle cavity.

its upper border to the inner surface of the mantle, close to
the origin of the latter from the body-wall. The inner
lamella of the outer gill is attached along the whole length
of its upper edge to the outer lamella of the inner gill.
The line of their junction is thickened and may be called
the axis of the gills. The axis is attached tor most of
its length to the ventral side of the body, but behind
becomes free. The inner lamella of the inner gill is
attached in front to the top of the toot, its middle portion
has a free edge, and behind the foot it is attached to its
fellow of the opposite side. The result of this arrangement
of the attachments of the gills is that the epibranchial
spaces, which are separate in front, join behind to form a

376

MANUAL OF ELEMENTARY ZOOLOGY

<cloacal space which communicates with the outside by the
dorsal siphon. Into this space opens the anus, which is
placed above the posterior adductor muscle. The gill
filaments (Tig. 280 B) bear strong lateral cilia (/.^.), which
set up a current ot water through the perforations of the
lamellae into the interlamellar spaces and so by way of
the epibranchial and cloacal spaces to the exterior at the
dorsal siphon. The current is maintained by the entry of
water at the -ventral siphon, where it passes over the
tentacles, which test its purity, causing if necessary a
sudden closure of the valves which drives water out at
both siphons and washes any obnoxious substance away.
The incoming current has a double function. It brings
•oxygen to the gills and mantle, which are the respiratory
■organs, and also carries the food particles, which some of
the gill-cilia ( l.f.c .) retain on the outer surfaces of the gills,
where others in certain definite tracts pass them on to the
labial palps. Particles rejected by the labial palps (p. 374)
are carried, by ciliary action over the inner surface of the
mantle to its edge, and there, from time to time, driven
•out by a sharp closing of the shell valves. The outgoing
current bears away carbon dioxide, faeces from the anus,
and excreta from the kidneys, which, as we shall see, open
into the inner epibranchial chambers at their front ends.

The swan mussel is a ccelomate animal, intermediate
■General between the earthworm and the crayfish in
Anatomy and respect to its coelom and haemocoele. It has
5ystimntary a Peri.visceral coelom, situated in the back,
enclosing the heart and rectum and com-
municating with the exterior by an excretory tube on
each side. 1 his space is the pericardial cavitv. In the
rest of the body the organs are separated by blood sinuses,
the circulation being an open one. Phe gonads repre-
sent a part of the coelom. Most of the viscera lie in the
upper part of the body, known as the visceral hump , but
the gonads and intestine lie in the soft region of the foot.
The mouth leads into a gullet, which passes upwards
into a moderate-sized stomach situated behind the
anterior adductor muscle. Into the stomach opens by
several ducts a large liver ” which surrounds it, whose
cells absorb soluble products of digestion and 111 an

THE SWAN MUSSEL

377

amoeboid manner take up minute particles for intracellular
digestion. The hinder end of the stomach communicates
on the right side with a closed groove of the intestine, the

ir.5.

Fig. 2 82. — Part of the dorsal side of a swan
mussel in which the pericardial cavity has been
opened.

a u., Auricle; d.s., margin of dorsal siphon; g., hinder
tips of gills, fused to form floor of cloacal chamber;
p.ad., posterior adductor muscle; pr.y posterior re-
tractor muscle; m., rectum; rp.o.y renopericardial
opening ; v.y ventricle ; v.s., margin of ventral siphon
(opened out by spreading the mantle).

Note between the posterior adductor muscles the fusion
of the mantle edges for a short distance, roofing in
the cloacal chamber just above the dorsal siphon.

caecum, which contains a transparent, gelatinous rod,
known as the crystalline style. This is composed of a
protein substance and projects into the stomach, where
it dissolves. Its function is to digest carbohydrates by

37^

MANUAL OF ELEMENTARY ZOOLOGY

aff.br.
,-eff. hr.

cl. ad .

urn.

Fig. 283. — Details of the anatomy of the swan mussel.

A, Diagram of a transverse section through the foot, showing the principal
blood vessels and, by means of arrows, the course of the circulation : each
gill is cut at an interlamcllar junction. — After Howes. B, the mouth and
neighbouring structures from in front ; C, a portion of the shell, in section ;
D, a plan of the nervous system from above, with the visceral commissures more
widely parted than they are in the animal.
a ad ., Position of anterior adductor ; aff.br., afferent branchial vessel ; au., auricle ;
B, glandular part of kidney ; c.p.c., cerebropedai commissure ; c.p.g., cerebral
(cerebropleural) ganglion ; eff.br ., efferent branchial vessels foot ; g., gills ;

inner labial palp; K.o., Keber’s organ; m., mouth; ml., mantle; ncr
nacreous layer; off, outer labial palp; p.g., pedal ganglion; pem., perios-
tracum ; pl.n , pallial nerve; prm., prismatic layer ; v., ventricle ; v.c., vena
cava ; v.cm., visceral commissure ; v.g., visceral (r»arietosplanchnic) ganglion.

THE SWAN MUSSEL

37 9

setting free a ferment which it contains. The intestine
starts from the lower side of the stomach, takes several
coils in the soft upper part of the foot, turns upwards,
and runs straight backwards in the middle line of the
upper part of the body to the anus. The straight part
of the intestine is known as the rectum. It lies in the

Fig. 2S4. — A semi-diagrammatic drawing of a trans-
verse section of the swan mussel in the region of
the hinder part of the foot.

cm., Auricle; B, glandular limb of kidney; B, non-glandular
limb of the same ; com., commissures between cerebral and
parietosplanchnic ganglia; ep.sp., epibranchial spaces;/,
foot ; gen., gonad ; i.g., inner gill ; i.l.j., interlamellar
junction ; il.sp., interlamellar spaces ; int., intestine ; m.f.,
muscular part of the foot; ml., mantle lobe ; o.g., outer
gill ; pm., pericardial cavity ; rm., rectum ; v., ventricle
v.c., vena cava.

380 manual of elementary zoology

pericardial cavity surrounded by the ventricle of the heart.
The ventral wall of the rectum is folded to form a longi-
tudinal ridge or typhlosole. Digestion in the intestine is
intracellular, in the cells of the epithelium and in white
corpuscles which pass through the epithelium to take up
food particles.

The kidneys or organs of Bojanus are two in number

Fig. 285. — The structure of Anodonta. — After Rankin.

«*,«., Anterior adductor ; c.p.g., cerebral or cerebropleural ganglia ;r/.,
stomach ; v., ventricle, with an auricle opening into it ; k., kidney,
above which is the posterior retractor of the foot ; r., rectum ending
above posterior adductor ; v.g., visceral ganglia with connectives (in
black) from cerebropleurals ; g., gut coiling in foot ; p.g., peda) ganglia
in foot, where also are seen branches of the anterior aorta and the re-
productive organs; l.p., labial palps behind mouth. At the posterior
end the exhalant (upper) and inhalant (lower) siphons are seen.

and lie side by side below the pericardium. Each is a
wide tube, doubled on itself, with one limb
excretory above the other, and the two ends together

conads in front. The lower limb has spongy walls

lined with a dark, glandular epithelium. It
opens into the front end of the pericardial cavity by a

THE SWAN MUSSEL

38 B

crescentic renopencar dial opening in the flooi . T he upper
limb has thin walls. It opens on the side of the body
into the adjoining inner epibranchial chamber, shortly
before the point at which the inner lamella ol the inner gill
becomes free. Thus the two ends of the kidne) tube cross,
the lower opening upwards and the upper downwards.
The organ is a coelomoduct. A pair ol glandular bodies
known as KebeP s ongans which he on each side in Iront ol
the pericardium store excreta in their cells. The} are
derived from the coelomic wall and, like the }ello\\ cells
of the earthworm, represent a portion of its epithelium

a. <to.

Fig. 286. A diagram of the pericardium and kidney of the swam

mussel, from the left side.

a.ao., Anterior aorta ; an., auricle ; B, glandular limb of kidney ; B , noil-glandular
limb of the same ; k.o., opening of the same ; p.ao., posterior aorta , pm.?
pericardial cavity; rp.o., renopericardial opening; rm., rectum; v., ventricle.

specialised for excretion. Coelomic fluid is. drawn by
ciliary action through the renopericardial opening into the
kidney and is discharged as urine by contractions of the
organ. The urine is of lower concentration than the
c celomic fluid or the blood so that solids must be re-
absorbed in the kidney, and the mussel, like other fresh-
water animals, keeps down its water content b\ excretion.
The principal nitrogenous excreta discharged through the
kidney are ammonia and amino compounds. The opening
of the" kidney has thick, yellowish lips. Immediately below
it is a somewhat larger opening with thin lips. I his-
belongs to the gonad, which is a branched structure lying,
in the upper part of the foot and alike in its general
structure in both sexes.

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MANUAL OF ELEMENTARY ZOOLOGY

The blood is colourless and contains white corpuscles.

The heart (Fig. 282) consists of a ventricle,
System^ which forms a jacket around the rectum,1 and
two auricles, which are triangular, thin-walled
structures, one on each side of the ventricle. From the
front end of the ventricle an anterior aorta passes forwards
above the rectum, and from the hind end a posterior aorta

Fig. 287. — A glochidium larva, as cast out from the parent, viewed

from behind. — From Latter.

passes backward below it. From branches of these the
blood passes into spaces between the organs. From the
foot and viscera it is gathered into a vena cava which lies
below the pericardium between the kidneys. Thence it

1 The ventricle of the heart of the swan mussel is really a wide, con-
tractile part of a dorsal blood vessel, which is continued forwards as the
anterior aorta. In various animals related to the mussel it lies altogether
above the gut, but here it extends downwards at the sides and encloses
it. A similar vessel is found above the gut in Annelida ( e.g . the earth-
worm) and in Arthropoda {e.g. the crayfish and cockroach). See p. 305.

THE SIVA AT MUSSEL

3*3

passes outwards through the kidneys to the gills, where it
circulates in irregular spaces in the inner parts of the
filaments. From these it is returned to the auricles. The
blood from the mantle returns direct to the auricles.

The nervous system comprises three pairs of ganglia
with commissures uniting them. The cerebral
ganglia are two small, orange-coloured bodies,
placed one on each side behind the mouth,
above which they are connected by a cerebral commissure.
They are sometimes known by the name “ cerebro-pleural/'

Nervous

System.

Fig. 288. — A glochidium larva in ventral view. — From Latter.

Byssus (cut short) ; d., future :mouth ; nt., adductor muscle ; s., sensory cells ;
t., main teeth and denticles on ventral edge of each valve.

because each contains the elements of two ganglia, the
cerebral proper and the pleural , which are distinct in
certain other bivalves and in whelks. The cerebral ganglia
supply the fore-part of the body, and each gives off a cere-
bropedal commissure to one of the two pedal ganglia which
lie side by side in the foot, just above the muscular region,
about one-third of the length of the organ from its front
end. The pedal ganglia bear the same relation to the cere-
bral that the subpharyngeal do to the suprapharyngeal in
the earthworm. The pedal ganglia give off several nerves
to the foot, and each sends a nerve to a statocyst which lies
shortly behind it. Each cerebral ganglion also gives off a

3S4

MANUAL OF ELEMENTARY ZOOLOGY

visceral commissure , which runs backwards between the
kidneys to join one of the visceral or parietosplanchnic
ganglia which lie as a fused pair on the under side of the
posterior adductor muscle, immediately within the skin.
The sense organs are inconspicuous. They include the
statocysts, the tentacles of the ventral siphon, a sensory epi-
thelium, believed to be olfactory, which covers the visceral
ganglion and is known as the osphradium , and tactile nerve
endings in various parts of the skin. There are no eyes.

The sexes of the swan mussel are separate. Sperm is

Life-History Passe(i out through the dorsal siphon and
spermatozoa are drawn into the females with
the inward stream. The eggs are fertilised in the cloaca!
chamber and then passed into the space between the lam-
ellae of the outer gill, where they develop. This takes
place in the summer. In the following spring the young
are set free. They are larvae, very unlike the parent, and
are known as glochidia (Figs. 287, 288). Each has a shell'
composed of two triangular valves, hinged along the base
and with the apex drawn out into a strong hook. There
is no posterior adductor muscle, anus, or foot, but in the
place of the latter is a gland which secretes a long sticky
thread known as the byssus , comparable with the threads
by which the adult sea mussel anchors itself. When
some small fish, such as a stickleback, passes over the
glochidium, the latter flaps its valves so as to drive out the
byssus, which if it touches the fish sticks to it. The
movements of the fish now bring the glochidium against
its body, whereupon the hooks are used to hold on to
its skin. The tissues of the fish become inflamed and
swell up, enclosing the little parasite, which lives for some
months by absorbing the juices of its host, during which
time it undergoes a change into the adult form. Eventu-
ally the skin enclosing the young mussel withers and it
drops off to lead an independent life. By means of this
larva, the slow-moving mussel is dispersed into fresh
feeding grounds by the fish, without the risk, which
would be considerable if so small a larva were free-
swimming, of being carried downstream to the sea. We
have seen that the young of the crayfish escape the same
danger by holding on to the body of their mother. It

MOLL USC A : THE SNAIL

385

is interesting to note that marine relations of both these
animals have free-swimming larvae, and to recall that
Hydra lacks the free larva of the sea-dwelling Obelia.
Soft-bodied, shelled, unsegmented, ccelomate animals

Moiiusca. a mantle> fo°t, and nervous system like

those of the swan mussel are known as
Moiiusca or true Shellfish. Snails and whelks ( Gastro-
poda), and cuttlefish ( Cephalopoda ) belong to this group.
These animals have heads, with eyes and a rough tongue
or radula , which are wanting in mussels, and their shell
is in one piece (or occasionally is lacking). The body of
a snail or whelk is flattened, not from side to side as in

v. h.

v.h. y

• /

/•

Fig. 289. — Diagrams of a mussel (A), a whelk (B), and a cuttle-
fish (C). — Partly after Lankester.

« , Anus ; foot ',/un., funnel through which water k squirted by the cuttle-fish tu
swimming; gut; A. .head; tnl.c., mantle cavity; sh., internal shell found
in some cuttlefish (the “cuttle bone”); ten., tentacles of the cuttlefish; v.k.
apex of visceral hump.

a mussel, but from above downwards. Its foot has a flat
sole, and the visceral hump, with the shell, is twisted.
The hollow axis of the spiral shell is known as the columella
and the animal is attached to it by a columellar muscle .
There is only one kidney and one auricle, which correspond
to those of the right side of the mussels. The pericardium
is small and encloses only the heart, and not the rectum.
The mantle cavity is represented by a deep sack, which lies,
not behind, where the main part of the mantle cavity is in a
mussel, but over the back, opening forward (Fig. 289, B).
In rotating to this position it has brought forward the
anus with it. In the snail, but not in the related whelks,
this sack is converted into a lung, with a narrowed opening
and a vascular roof, and the gills are lost. Snails also

25

3S6

MANUAL OF ELEMENTARY ZOOLOGY

MOLL USC A: THE SMALL

387

differ from mussels in having the ganglia concentrated
into a clump around the gullet (the cerebral above,
pedal, pleural, and visceral below), and a complicated,
hermaphrodite reproductive system, which opens far
forwards on the right. The snail has one gonad, which
produces, in the same follicles, sperm during most of the
year, but ova for a short period in the summer. The
“ hermaphrodite duct ” which leaves the gonad presently
reaches a gland by which a coat of albumen is provided
for the ova ; after this the common duct has two channels
one for the ova, the other for the sperm. These channels
eventually separate, but meet again at the genital opening.
There is a penis, with a “ flagellum ” in which is formed a
slender spermatophore or sperm packet ; for the reception
of the spermatophore of the partner a long spermatheca
is provided. Impregnation is reciprocal (Fig. 458), and
just before it each partner drives into the other a sharp
calcareous “ love dart,” which is supposed to impart a
sexual stimulus. The eggs, which are laid in the earth,
have albumen and chalky shells. The young, at birth,
resemble the adult. The snail is a vegetable feeder, rasping
off portions of the tissues of plants between a horny upper
jaw and the rows of horny teeth which roughen its tongue.
The alimentary canal begins as a “ buccal mass,” which

Fig. 290. — Anatomy of the edible snail, Helix pomatia.

A , View from the right side ; B, the same after removal of the shell, part of the
mantle, and the upper part of the spiral visceral hump ; C, dissection ; D,
section through buccal mass, enlarged.

1, Shell (note lines of growth) ; 2, foot ; 3, mantle ; 4, anterior tentacle ;
5, posterior tentacle, at the end of which lies a retractile eye ; 6, edge of mantle
(“collar”); 7, opening of lung ; 8, anus ; 9, common genital opening ; 10,
mantle cavity or lung ; n, dorsal wall of body (floor of lung) ; 12, pulmonary
vein ; 12', plexus of pulmonary vessels from which pulmonary vein collects;
13, rectum; 14 ureter; 15, kidney ; 16, auricle ; 17, ventricle ; 18, pericardium’;
the renopericardial opening (not shown) is near the end of this index-line ;
kidney, pericardium, and heart lie in the hinder part of the roof of the mantle
cavity ; ureter and rectum run along its right side ; 19, buccal mass, which
contains radula ; 20, oesophagus; 21, crop; 22, left salivarv gland; 23,
stomach; 24, right liver; 25, left liver; 26, intestine; 27, ovotestis ; 28,
hermaphrodite duct ; 29, albumen gland ; 30, male part of compound genital
duct; 31, female part ; 32, vas deferens ; 33, “ flagellum ” of penis ; 34, penis
(protrusible) ; 35, oviduct; 36, spermatheca; 37, “ mucous ” glands of uncertain
function; 38, sac of “love dart”; 39, vagina; 40, cerebral ganglia; 41, pedal
ganglia ; 42, viscero-pleural ganglia ; 43, radula (note, behind it, the sac in
which it grows, and is pushed forward as it wears away in front) ; 44, jaw ;
45, pedal gland, which secretes the slime of the snail’s track.

388 MANUAL OF ELEMENTARY ZOOLOGY

contains the buccal cavity and the radula with its muscles,
and continues as oesophagus, crop, stomach, intestine, and
rectum ; digestion of carbohydrates takes place in its
cavity by means of fluids secreted by a pair of salivary
glands and by the paired liver. The latter, like the liver
of the swan mussel, is also the seat of absorption and of the
digestion of proteins, which is intracellular. The excreta
comprise uric acid as well as urea and ammonia. The
blood contains haemocyanin (see p. 306). In the winter
the animal retires to some sheltered spot and closes the
mouth of its shell by a disc, the epiphragm , which it
secretes from the edge of the mantle. The anatomy ot
the snail is displayed in Fig. 290, and further details
concerning it are stated in the explanation of the figure.

In a cuttlefish the body is flattened from before back-
wards, and the foot forms a funnel, the squirting of water
through which from the mantle cavity causes the animal
to move in the opposite direction. The sucker-bearing
tentacles which surround the mouth are said also to
represent part of the foot. There are two feather-like
gills ; and an ink-gland, which opens with the rectum into
the mantle cavity, enables the animal to cloud the water
behind it in escaping from its foes. In some cuttlefishes,
as in the Squid (Sepia), the shell is present as an internal
vestige, the “ cuttle bone ” ; others, as the Octopus, lack
it altogether. Only in the Nautilus is it well-developed
and external.

CHAPTER XIX

THE STARFISH. ECHINODERMS

One of the most familiar animals of the seashore is tne
Common Starfish, A s ter i as rube ns. It may

Features. often be f°und dead or dying upon the beach
where it has been thrown up, or living in pools,
but its principal haunts are in somewhat deeper water.
For all its seeming helplessness, it is an .exceedingly
voracious animal, and is particularly destructive to shell-
fish, so that it is a pest in oyster beds. Its body, of a
colour varying from orange to purplish and darker above
than below, has the shape of a star, with five tapering
rays, or arms, meeting in a central region known as the
disc. The upper side is called aboral , the lower oral ,
because on it, in the middle of the disc, lies the mouth.
The direction of each arm is known as a radius. The
region between two arms is an interradius. Along each
arm runs on the oral side a deep ambulacral groove , and
the grooves meet around the mouth, which has a mem-
branous lip or peristome. The surface of the body is
soft and ciliated, but below it is a tough body-wall,
strengthened by a meshwork of rod-shaped, calcareous
ossicles , which can be felt and seen through.it. Over the
interspaces between the ossicles the skin is raised into
delicate, hollow outgrowths, the dermal gills , into which
the body cavity is prolonged. From the junctions of the
ossicles arise blunt spines , each of which is surrounded by
a cushion of skin. Crowded upon these cushions, and
scattered between them, are remarkable little organs known
as pedicellariat , each of which is a minute pair of pincers,
supported by little ossicles, one at its base and one in each
jaw. The pedicellariae are defensive organs. They are

389

390

MANUAL OF ELEMENTARY ZOOLOGY

of two kinds, a smaller kind, found upon the cushions of
the spines of the back, in which the supporting ossicles
cross at the base like the blades of a pair of scissors, and
a larger kind, scattered between the spines, whose ossicles
do not cross. In an interradius, on the aboral surface.

Fig. 291. — Part of an aboral view of a starfish ( Asterias rubens).

Fig. 29 i a. — An enlargement of a small part of the surface of

the same.

g., Gills ; peel., large pedicellaria ; ped'., small pedicellaria ; sp., spine.

is a conspicuous button-like ossicle, covered with fine
grooves, and known, from its coral-like appearance, as
the madreporite. Its grooves are pierced with fine pores
through which, by the action of cilia, water is drawn in.
The anus is a small opening, almost in the middle of the
aboral surface, but slightly displaced towards the inter-

THE STARFISH . ECHINODERMS

39i

Fig. 292.— Parts of an oral view of a starfish (. Asterias rubens).

a, An arm with tne ambulacral groove widely open ; b, an arm with the ambulacra!
groove closed by the contraction of its sides and the bending over of the adam-
bulacral spines ; c, an ambulacra) spine, with its tuft of large pedicellariae,
enlarged.

392

MANUAL OF ELEMENTARY ZOOLOGY

radius next (clockwise) to that in which the madreporite
lies. Each ambulacral groove is crowded with tube-feet ,
delicate, cylindrical tentacles, ending each in a sucker, set
in four rows. It is by these that the animal crawls, and
they are, also used to hold prey. At the sides of the ambu-
lacra! grooves stand a number of adambulacral spines ,
which bear pedicellariae of the uncrossed kind and can be

brought together over the groove so as to protect the tube-
feet. At the bottom of the groove a longitudinal nerve
ridge marks the position of a radial nerve cord. At the
end of the groove is a single sense-tentacle , like a tube-foot
but smaller and without sucker, which subserves the
olfactory sense and has at its base a red eye-spot.

The body-wall is covered by a ciliated, columnar epi-
Body-wafi thelium, which contains gland cells and sense
Nervous ’ cells. , , Among the bases of the cells lies a
udcaiom. dense tangle of fine nerve fibrils, some of
which start as processes of the bases of sense
cells, while others belong to nerve cells imbedded in the
tangle. Above the nerve ridge around the mouth and
down each arm, this plexus is thickened to form a special
conductive system in the form of an oral nerve ring with
radial nerve cords , which send branches to the tube feet
and end in the sense tentacles. Many of the fibrils in this
system are arranged to run in the directions of its strands.
Under the peritoneal epithelium is a similar but slighter
system which communicates with the ectodermal system
and consists of motor fibres for the muscles of the large
ossicles and body- wall. Thus the nervous system is in a
more primitive condition than that of any other animal
we have studied, except Hydra. Below the epithelium,
the body-wall is composed of connective tissue, in which
the ossicles are imbedded. The deeper part contains some
muscular fibres running in various directions. On opening
the body, there is revealed a spacious cavity, the peri-
visceral coelom , which extends into the arms, and contains
the alimentary canal .and generative organs. The coelom
is lined by a ciliated peritoneal epithelium, and along the
oral side of each arm, where the body-wall roofs the
ambulacral groove, runs a ridge, the ambulacral ridge ,
caused by the projection of a double row of large, trans-

THE STARFISH . E CH1N0DERMS

393

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MANUAL OF ELEMENTARY ZOOLOGY

versely placed ambulacral ossicles. At the outer ends of
these, alternating with them, lie smaller adambulacral
ossicles j and upon the adambulacral ossicles stand the
adambulacral spines. In each interradius a stiff interradial
septum, projects into the coelom between the arms. To the
septum which is situated in the interradius of the madre-
porite there is attached a sack, the axial sinus , also a part
of the coelom, and in this are lodged the stone canal , which,
as will presently be stated, runs downwards from the
madreporite, and a spongy, brown organ, the axial organ,
to which also we shall return.

Alimentary

Canal,

Feeding, and
Excretion.

1 he mouth opens through a short oesophagus into a
great sack, the stomach , which has in each
interradius a wide pouch, attached to the
ambulacral ridge by two retractor muscles.
Above, the stomach communicates by a wide
opening with a five-sided pyloric sac, each angle of which
is prolonged into a tube or pyloric duct, that runs into an
arm, and there forks into two branches, the pyloric cceca ,
each beset with numerous little pouches and slung from
the aboral wall of the arm. From the pyloric sac a short,
conical rectum leads to the anus. It bears interradially
two small brown branches, the rectal cceca. The star-fish
will eat any animal that it can master. Small prey may
be taken into the mouth, but usually digestion is per-
formed in a remarkable manner outside the bodv, the
arms bending round the prey and holding it with their
tube feet, while the stomach is forced out, by contraction
of the body- wall compressing the coelomic fluid, enwraps and
digests the prey, and is afterwards withdrawn by the re-
tractor muscles. Bivalves, which are a great part of the
food, are opened by arching the body over them and
parting their valves by the pull of the tube feet, and the
stomach is then inserted into the shell. The digestive
juice is secreted by the cells which line the pvloric cseca.
Shells are left behind by the stomach, or rejected through
the mouth, very little matter being cast out through the
anus. Excreta, which contain ammonia and amino com-
pounds and some urea, are got rid of through the walls of
the gills, partly in solution, partly as granules carried by
amoeboid cells which pass through to the exterior. The

THE STARFISH. E CHIN ODE R MS

395

Fig. 2Q4.— Parts of the aboral half of a starfish ( Asterias rtibens),
removed, with the alimentary canal, from the rest of the body,
and viewed from within. One lobe of the stomach has been
cut away, and another partly turned back. The detached figure
represents an enlarged view of the axial sinus and adjoining

structures.

ah muse., Aboral muscle; ax.o., axial organ; ax.s., axial sinus; 1st., one of the
lobes of the stomach; py.c., pyloric emeum ; py.d ., pyloric duct ; pyx pyloric
sac; r.c., rectal cacum; sep., septum; st.c., stone canal.

396

MANUAL OF ELEMENTARY ZOOLOGY

rectal caeca appear also to excrete, waste. matter, .which
passes 'out by the anus.

Regulation of the water content of the animal is in normal circum-
stances unnecessary because the optimum concentration of the body
fluids is the same as that of sea water, which varies very little.
Accordingly the starfish has no power of such regulation. On this
account it is unable to survive in fresh or brackish water. Such
animals are said to be stetiohaline. Animals that can endure great
changes in salinity, as the salmon does, are euryhaline.

The working of the tube feet is brought about by a

peculiar system

Water

Vasiular
Systems.

of tubes, derived
during develop-
ment from the
coelom, known as the water
vascular system. This starts
at the madreporite as the
stone canal , so called because
it is strengthened by cal-
careous matter. The wall of
this canal makes a curious
projection into it, Y-shaped in
section, with the arms of the
Y rolled, so that the surface
is greatly increased, and it is
Fig. 2Q5. A diagram of the ciliated. The lower end of

stofishVaSCUlar SyStem °f 2 the stone canaI j°ins a
. ' , canal around the mouth,

y Ampulla ; tyicui.. ? madreporite! i ■» • ,

r.w.v., radial water vessel ; st.c., above the peristome, and

a radial canal
runs down each arm, below
the ambulacral ossicles. From the radial canals small
transverse canals run, one to each tube foot. The
hollow of the tube foot is prolonged inwards into a
bulb called the ampulla , which projects into the coelom
of the arm. The transverse canal joins it just below the
ampulla, by a valved opening which prevents fluid from
flowing into the radial canal, so that, when (by a circular
muscle layer) the ampulla contracts, the fluid in it stretches
out the tube foot. Divers muscles in the foot direct it
against the ground, by cupping the sucker cause it to adhere,

stone canal; t.f.v , vessel of tube frnrn fUa
foot; w.v.r., water vascular ring.

THE STARE /HI. ECHINODERMS 397

and then shorten the loot, so that it tends to draw the body
forward. Owing to the shortening of the foot the fluid passes
back into the ampulla. The pressure of the fluid in the
water vascular system is regulated by gain, and perhaps also
by loss, of water through the madreporite.

Like most animals, from the Amoeba to man, the starfish moves
towards the side from which it has received a slight
Behaviour. stimulus but away from one strong enough to be caused
bv some event with which the creature might find it difficult to cope.
In crawling one (or sometimes two) of the arms is directed forwards.
On this leading arm each tube foot— not moving in step with any
other but quite independently — is extended in the direction of the
arm, takes hold with its sucker, shortens so as to take part in pulling
the arm forwards, swings back, lets go with a slight kick so as to
push it on, and then swings forwards to take another such step. On
the other arms the tube feet behave in the same manner but swing to
and fro in the direction in which the animal is crawling— that is,
more or less transversely to their arms. A starfish which falls on its
back can right itself. In this process also one or two arms take the
lead, turning over so as to touch the ground and holding on with
their tube feet, while the other arms, probably stimulated b} the
first movers, arch over by muscular action till the creatuie topples
over on to its oral side. The arms which take the lead are those
which had previously led in crawling. Pedicellariae bend towards the
site of a gentle stimulus on the skin, opening their blades and closing
them upon any object that comes against their inner sides. The
larger kind are set in motion by a weaker stimulus, than is seeded to
move the smaller ones ; these are brought into position tor action by
the rising of the cushions upon which they stand. It the ambulacral
groove be touched the adambulacral spines come together over it.
We have already seen how the animal feeds. . . . .

The nature of the nervous processes by which these activities are
brought about will appear if we examine the course of events in
locomotion. A stimulus which is not too strong affecting an arm
sets up an impulse which, spreading through the nerve net and
reaching to a distance by means of the radial “ nerve, causes the
tube feet of the arm to extend towards its tip. Thus they make
contact with the ground in front of them, and this sets up in them a
reflex which makes them step in the way we have described. I he
arm being thus pulled forwards drags the rest of the body after it,
so that the tube feet on the other arms make contact with the ground
on the side towards which the leading arm is moving and thus they
step in the same direction. A strong stimulus affecting an arm
contracts the tube feet instead of extending them, and reaching the
other arms along the nerve ring and radial nerve, has there the same
effect. But, as we saw in Hydra , excitation through a nerve net
weakens as it travels, so that the arms on the opposite side are the
least stimulated and are the first to recover. On doing so their tube
feet extend and so make contact with the ground and start stepping

398 MANUAL OF ELEMENTARY ZOOLOGY

away from the stimulus. All muscular activity in the starfish is
brought about essentially in the same manner as its locomotion.
Tube feet are extended, spines moved, pediceliariae opened, or large
muscles contracted by impulses reaching them through the nerve
nets. “ Nerves ” enable these impulses to come from a distant point
which has become dominant by being stimulated. Stepping of the
feet or closing of the pediceliariae or protrusion of the stomach when

Fig. 296. — Diagram of a transverse section of the arm of a starfish.

ab. muse., Muscle which straightens the arm; ad oss., adambulacral ossicle;
ad.sp ., adambulacral spine; amb.oss., ambulacral ossicle; amp., ampulla'
of tube foot; muse'., muscle which opens the ambulacral groove; ped.,
pedicellaria ; r.b.v., radial “blood vessel”; /./., tube foot. Other lettering
as in Fig. 254.

food stimulates the mouth are reflexes performed by means of stimuli
received in the organ themselves. The whole procedure is much like
that in the nerve net of Hydra ; only in the starfish the transmission
of impulses from the temporarily dominant region to distant parts is
improved by the system of “ nerves/'* and there are definite and
complicated local nervous mechanisms. Although these features
enable the animal to react as a whole more promptly and completely
to a stimulus there is no permanent dominance of one part organised
for that purpose. The starfish shows what can be achieved by the
nerve net ; it also shows the slowness and vagueness of action which

THE STARFISH. ECHINODERMS

399

is inevitable in the absence of a brain. Its lack of that organ is due
to its radial symmetry, which provides no suitable site for one.

pe rist.

Fig. 297. — Part of a view from above of the oral halt of a starfish
(As ter: as rubens), after removal of the alimentary canal.

xmb.r., Ambulacral ridge; amp., ampullae of tube feet; aar.s., axial sinus,
with stone canal and axial organ; gon., generative organ*, perist peri-
stome; ret.m retractor muscle of the stomach; sep., septum; T.b.,
Tiedemann’s body.

The radial water vessel in each arm lies close under the
ambulacral ossicles ; below it there is a c celomic space,
roughly diamond-shaped in transverse section, which is
known as the radial perihcemal cavity . Below the

400

MANUAL OF ELEMENTARY ZOOLOGY

perihaemal cavity the epidermis is thickened by an in-

andPseudo crease in the nerve plexus, and folded so as
h*maS|6U °' to project into the ambulacral groove as the
Systems. nerve ridge. . Round the mouth, the radial
perihaemal vessels are joined by an oval pevihcBvnal vivig d

Fig. 298. — The Bipin-
naria iarva of a starfish,
in ventral view.

an., Anus; b., postoral ciliated
band ; b' ., preoral band ;
m. mouth.

The depressed region between
the ciliated bands is shaded.

Fig. 299.— Diagrams to show the
relative extent of the oral and
aboral surfaces, and to compare
the form of body, in the several
classes of Echinodermata. The
diagrams are in the same morpho-
logical position.

1, Asteroidea; 2, Ophiuroidea; 3, Echino-
idea; 4, Holothuroidea ; 5, Crinoidea.

ab., aboral surface; or., oral surface.

Each such radial vessel is divided longitudinally by a
vertical septum, and in this septum lies a strand of a
peculiar tissue which in the starfish takes the place of the
blood vessels. This is a part of the connective tissue in

1 Adjoining this is another coelomic tube, the so-called “inner
perihaemal ring,” which is connected not with the perihaemal vessels
but with the axial sinus.

THE STARFISH. ECHINODERMS

401

which the fibres are more sparse and the ground substance
more fluid than elsewhere, and it is believed that along the
strands which are formed ol it substances diffuse, and
amoeboid cells wander, more readily than elsewhere.
Around the mouth a ring strand joins the radial strands,
with this is connected the tissue of the axial organ, and
with the aboral end of the axial organ is again connected
an aboral ring, from which strands extend to the generative
organs.

A i)

Pig. 300. — Semidiagrammatic views of a starfish (C), a sea urchin (B)v
a holothurian (A), and a crinoid (D), in the natural position. —
From Lang.

a, Ab. ’nil side; o, oral side.

The axial organ, however, is primarily of importance,
not as a part of this “ pseudohaemal system,”
Reproduction, as tpe orfginal seat of the genital cells,

for which reason it is often known as the “ genital stolon,”
while the aboral ring is the “ genital rachis.” Along the
latter the genital cells wander from the stolon to the actual
gonads. These are ten in number, shaped like bunches
of grapes, and varying in size with the season of the year.
They are attached to the body-wall by their ducts, which
open one on each side of the base of each arm, towards
the oral aspect. The sexes are separate, but do not differ
externally. Eggs and sperm are shed into the water,
where fertilisation takes place. The cleavage of the ovum
is complete (“ holoblastic,” p. 637) and practically equal.
It leads to the formation of a remarkable, bilaterally sym-
26

402

MANUAL OF ELEMENTARY ZOOLOGY

metrical larva (the Bipinnaria , Fig. 298), which swims by
two bands of cilia. This, after passing through a fixed stage
(Fig. 301), gives rise to the radially symmetrical adult,

Fig. 301. — The larva of a starfish at the fixation stage, viewed
from the right-hand side. — From MacBride, after Johannes
Muller.

The Bipinnaria larva has passed into a stage known as the Brachiolaria, by the
development of three fixing processes at the anterior end. Ast., rudiment of the
future body of the starfish ; b., postoral ciliated band ; b', preoral band ;
fix., fixation processes at the anterior end of the larva.

The larva is seen from the right-hand side, and its position is inverted from that
of Fig. 298

through a peculiar metamorphosis, in which its left and
right sides become the oral and aboral surfaces.

It is evident that the Starfish is a peculiar animal, which
differs greatly from any of those that we have studied
hitherto. It is tripoblastic and coelomate, but with-

THE STARFISH. ECHINODERMS

403

out blood vessels. It has an exceedingly complex system
of coelomic spaces, part of which subserves the
mata!°der* working of the altogether peculiar tube feet.

It forms in its mesoderm calcareous ossicles, but
these are quite unlike bone in fine structure. Its nervous
system is of a grade not much above that of the Coelenterata.
It is radially symmetrical, but starts life as a bilaterally
symmetrical larva. Animals which share with it these
peculiarities are known as Echinodermata (Figs. 299, 300).
To them belong : the Starfishes ( Asteroidea ) ; the
Brittle Stars ( Ophiuroidea ), whose arms are slender,
mobile, and muscular, arise abruptly from the disc, and
contain no pyloric caeca, whose madreporite is on the
oral side, and whose nerve cords are covered over ; the
Sea Urchins ( Echinoidea ), whose ossicles form a plate-
armour in the wall of the body, which latter is swollen
and without arms, so that the rows of tube feet ( ambulacra )
run meridionally over the surface, where the nerve cords
are enclosed as in the Brittle Stars ; the Sea Cucumbers
(. Holothuroidea ), which are soft-walled sea-urchins, drawn
out, from mouth to anus, into a sausage-shaped body ;
and the Sea Lilies ( Crinoidea ), which are starfishes with
branched arms and with suckerless “ tube feet,” fastened
either temporarily, in a post-larval stage, or permanently,
by a stalk upon the aboral surface, while the anus lies
upon the oral side.

The fixation of the Sea Lilies, and the fact that star-
fishes are fixed when the bilateral symmetry
Symmetries. Qf }arva changes to the radial symmetry of

the adult, are interesting facts in view of the fixation
which is general in the other great group of radially
symmetrical animals, the Coelenterata. Radial symmetry
is essentially the symmetry of a sessile animal, which is
in the same relation with its surroundings on all sides,
whereas bilateral symmetry is that of a travelling animal,
which needs differentiation of the upper side from that
which faces the ground, as well as of the fore from the
hind end. It is likely that at one time all echinoderms
were fixed, and that those which are now free retain the
radial symmetry of their sessile ancestors.

CHAPTER XX

THE LANCELET. CHORDATA

Habits and

External

Features.

The Common Lancelet, Amphioxus lanceolatus , is a little,
fish-like creature found on most . European
coasts, including those of Britain, living in shal-
low water on a sandy bottom. It passes most
of its time buried in the sand, with its length
upright and the fore end projecting, gathering small
organisms for food by a ciliated apparatus around the
mouth. From time to time, usually at night, it leaves the
sand, and then shows that it can swim swiftly by movements
of its muscular body. It is about an inch and a half long,
lustrous but translucent, slender, pointed at each end, and
flattened from side to side. The head is in no way marked
off from the rest of the body, and there are no ears, nostrils,
or limbs. A low dorsal fin runs along the middle of the
back from end to end, becoming deeper at the hinder end
as the upper lobe of a caudal fin , which passes round the
end of the tail The under lobe of this is continuous with
a low, median ventral fin which extends along the hinder
third of the body.- In front of the ventral fin the belly is
flattened and bears at each side a continuous lateral fin or
metapleural fold . At the narrow front end ( rostrum ) the
dorsal fin passes round the tip of the body and runs back
below to become continuous with the right side of the oral
hood (see below). The sides of the body are marked by a
series of about sixty v-shaped lines, with their apices
forwards, due to septa of connective tissue known as
myocommata, which divide the muscles of the body-wall
into segments called myomeres. Certain of the internal
organs are repeated in correspondence with these, so that
the body is segmented, though not so completely as that of

the earthworm. The segmentation is peculiar in that the

404

THE LANCELET

405

myomeres of opposite sides alternate. About seven
myomeres lie in front of the mouth. The anus lies against
the left side of the ventral fin where that fin passes into the

Fig. 30 2,—AmpAioxus, from the left side, with the atrial floor

contracted.

Fig. 304. — The same, from the ventral side, after the floor of the

atrium has been cut open.

a;; Anus; at.fi., floor of atrium; al.fi’., the same cut through and turned back;
Up., atriopore ; atp'., line indicating position of same in side view ; c./., caudal
nn * d. f.r., rays of dorsal fin ; est., endostyle ; gon., gonads ; lr., liver , m-J->
metapleural fold ; mym., myomeres ; myc., myocommata or septa of connective
tissue between the myomeres ; n.c., nerve cord ; nch., notochord , or.c., oral
cirri ; or.h., oral hood ; ph., pharynx ; v.f.r., rays of ventral fin.

caudal fin. Behind it is a region of the body of some length
which does not contain any part of the alimentary canal.
Such a region is known as a tail. At the end of the flat
region of the belly is a mid-ventral opening known as
the atriopore , through which a current of water escapes.

406

MANUAL OF ELEMENTARY ZOOLOGY

Below the pointed front end is a cavity, the buccal cavity
or vestibule , surrounded by an oral hood , the edge of
which is beset with slender, ciliated tentacles or cirri.
At the hinder side of the vestibule is a muscular partition,
known as the velum , whose opening is bordered with
about a dozen velar tentacles . On the inside of the hood
a lobed tract of epithelium, which bears long cilia and is
known as the wheel organ , encircles the vestibule just in
front ot the velum ; between its two main branches is a
median pit, known as Hatschekl s pit. The opening of the
buccal cavity is the mouth. The opening in the velum has

Fig. 305. — A view from the left side of the region around the atriopore
of a specimen of Amphioxus with the atrial floor expanded.
Lettering as in Figs. 302-304.

also often been called the mouth ; it is better named the

enterostome.

The atriopore leads from a large space which lies below
. and at the sides of the middle part of the bodv

and is known as the atrium, this space is not
really within the body, but is enclosed between the body
and two longitudinal folds of the body-wall, like those
which form the branchiostegites of the crayfish, save that
they meet in the middle line below, leaving at their hinder
end an opening which is the atriopore. The atrium
communicates with the pharynx by a number of slits
at each side, known as the gill slits , separated by narrow
gill bars , and a current of water which is passed into the

THE LANCE LET

407

mouth by the cilia around it is caused by cilia on the gill
bars to flow through the slits into the atrium, and so out
at the atriopore. The atrium is prolonged backwards on
the right side behind the atriopore almost as far as the anus.

The skin is covered with a columnar epithelium, ciliated
only within the oral hood and in parts of the
atrium. The connective tissue is scanty, and
consists ot nbnllated ground substance with
some cells. There is a thick muscular body-wall, divided,

4-f r-

1

Fig. 306. — Amphioxus. The forepart of the body cut in half

longitudinally.

at., Atrium; atrial floor; c.c., central canal of nerve cord; c.v., cerebral

vesicle ; d./.r., dorsal fin rays ; est., endostyle ; n.c., nerve cord ; nch notochord ;
or.c. , oral cirri ; or.h., oral hood ; p.ph.b peripharyngeal band ; pg., anterior
pigment spot; ph., pharynx; sk.c., skeleton of cirri ; sk.r., skeleton ring in oral
hood ; v.t., velar tentacles; vrn., velum ; w.o., part of wheel organ.

as we have seen, into segments, which are V-shaped and
fit into one another so that several are cut in a transverse
section. Within the body-wall lies a perivisceral coelom,
not divided by septa, but greatly complicated by the
presence of the gill slits, which reduce it in the region of
the pharynx to a number of canals presently to be described.
There are numerous other coelomic cavities, of which
the most important are those in the region in front of the
mouth, in the velum, in the metapleural folds of the

4o8 MANUAL OF ELEMENTARY ZOOLOGY

larva,1 and in the gonads. As in the frog, the dorsal body-
wall is much thicker than the ventral. In it there lies alongi-
tudinal, hollow central nervous system, comparable to that
of the frog, but at the front end not enlarged into a brain,
though the cavity, which behind is narrow like the central
canal of the frog’s spinal cord, is in front wide like that of the
frog’s brain. Below the central nervous system, along the

Fig. 307. — A , the hinder end of the pharynx of Amphioxus , from the

left side, highly magnified ; B, a diagram of the mode of origin ot

the gill clefts.

©

Endostyle ; pr.b ., primary bar ; svm., synapticulae ; t.b., tongue bar ; 2 'si., the
two secondary gill slits which arise from a primary slit. The skeleton is
shown in black.

whole length of the body, lies an elastic rod, the notochord ,
derived from the roof of the gut in the course of develop-
ment and bound to the nerve cord by a connective tissue
sheath which surrounds them both. In front it extends
beyond the nerve cord. There is no skeleton of bone or
cartilage, but stiff rods of organic material support the
gill bars and cirri, and gelatinous “ rays ” the dorsal and
ventral fins.

1 The metapleural canals of the adult are perhaps not c celomic.

THE LANCELET

409

The opening in the velum leads into a wide cavity
known as the pharynx, which forms about half the

est. exoe.

Fig. 308. — A transverse section through the pharyngeal region of

Amphioxus .

atr., Atrium ; b.t., brown tube ; d.coe., dorsal coelom ; d.fr., dorsal fin ray ; e.coe.,
endostylar coelom, containing ventral aorta ; e.gr., epipharyngeal groove ;
est., endostyle ; g., gonad ; hep.v., hepatic vein (here a plexus) ; l.coe., coelom
around liver ; lr., liver ; m.c., metapleural canal ; mym., myomere ; nch., noto-
chord ; sbr.a., suprabranchial artery or paired dorsal aorta ; sp.c., spinal cord.

length of the gut, and is placed in a portion of the body
which is enclosed by the atrium. Its sides are pierced by

4io

MANUAL OF ELEMENTARY ZOOLOGY

the gill slits, which lie obliquely, their lower ends being
behind the upper, so that a number of them
are cut in a transverse section of the body.
Each gill bar is covered with a deep columnar
epithelium, ciliated except on the side towards
the atrium, and contains a skeletal rod. At the tops of
the bars these rods fork and join one another over the
arches. At the lower ends the rods of alternate bars fork

Alimentary
System and
Perivisceral
Cavity.

ph

I

Fig. 309. — Transverse sections of gill bars of Amphioxus.

— Partly after Benham.

at., Atrial side ; at.ep., atrial epithelium ; bl., main blood vessels ;
bl'., capillaries ; cil.ep., ciliated epithelium ; coel., coelom ;
f.c., frontal cilia ; lx., lateral cilia ; ph., pharyngeal side ; pig.,
ment cells ; sW., skeletal rods ; sk' ., additional skeletal piece.

but do not join their neighbours, which are unsplit. The
bars which contain forked rods are known as primary bars,
the alternate bars, with unsplit rods, are secondary bars , or
tongue bars, because they arise in development by the down-
growth of a tongue-shaped process from the top of a slit,
thus dividing it into two secondary openings which become
the permanent slits. This process may be seen in all its
stages at the hind end of the pharynx, where new slits are
continually being added as long as the animal is growing.1

1 In the larva the primary slits correspond with the myomeres, but
afterwards they become more numerous.

THE LANCE LET

4ii

The rods of the primary bars are really double, consisting of two
strips which lie side by side touching one another in the bar, but

Fig. 310.-- A transverse section of Amphioxus in the region of the

intestine.

C.s., Connective tissue sheath of the notochord ; coel., coelom ; d.ao., dorsal aorta J
d.f.r dorsal fin ray ; d.r., dorsal root ; int., intestine ; mym., myomeres ; n.c.,
nervecord ; nch notochord ; pr.at backward prolongation of the atrium ; s.i.v.,
subintestinal vein (here represented by several vessels); v./-r-i ventral tin ray.

separate to form the forks above and below. The rods of the secondary
bars are single, though each parts into two to become forked at its
upper end.

412-

MANUAL OF ELEMENTARY ZOOLOGY

The gill bars are connected by horizontal bars or synapti-
culce , of which two or three cross each slit. These contain
skeletal rods. Along the mid-dorsal line of the pharynx is
a deep epipharyngeal groove lined by ciliated cells. The
mid-ventral wall is formed by a longitudinal bar with which

A lateral view of the upper region of the pharynx, the body-wall being removed.
The atrial chamber is laid completely open by the removal of its outer wall,
which is cut through along its line of insertion. The result is to show that
the chamber is prolonged dorsally into a series of bays (6.), which lie
outside the tongue bars p.£.). Into these bays the nephridia («.) open by
pores (o.), while they also project internally by blind funnels (/•), fringed
by very large solenocytes (c.). The bays are separated by ridges. (<Z), formed
by a downgrowth of the walls of the coelom over the primary bars 0*.);
my.y a myomere; sy., one of the synapticulae connecting the pharyngeal
bars

the lower ends of the gill bars are fused. This bar bears a
groove known as the endostyle , lined by ciliated cells with
two longitudinal bands of gland cells on each side.
These cells secrete mucus which is passed outwards by
the cilia. At the front end of the pharynx two peri-
pharyngeal bands of long cilia connect the endostyle with
the epipharyngeal groove. Each primary bar contains a

THE LANCET ET

4i3

narrow coelomic canal. At the tops ot the bars the
c celomic canals of each side join a longitudinal canal,
known as the dorsal ctelorn, which hes within the body-
wall above the atrium. Below the bars the^ canals join a
median longitudinal subendostylar coelom. This arrange-
ment is the remains of the
originally continuous perivisceral
coelom of the pharyngeal region
of the body, broken up into
canals by the piercing of the walls
by the gill slits. Behind the
atrium, the perivisceral coelom is
a narrow but' unbroken space
and surrounds the alimentary
canal, which runs straight back
through it to the anus. Im-
mediately behind the pharynx
is a very short, narrow oesopha-
gus, continuous with the epi-
branchial groove ; then comes
a wider region known as the
stomach. This gives off forward
on its right side a pouch known
as the hepatic c cecum or liver,
which pushes before it the atrial
wall and lies in the atrium be-
side the pharynx. Behind the
stomach the gut narrows and is
known as the intestine. The
whole alimentary canal is lined
by a columnar epithelium which
contains gland cells and is
ciliated in tracts.

Fig. 312. — The outlines
of a transverse section
through the pharyngeal
region of Amphioxus ,
with arrows to show
the course of currents
in the pharynx and
atrium. Heavy arrows
show the main current
passing from pharynx
to atrium, light arrows
show the currents
which transport to the
epipharyngeal groove
mucus containing the
particles retained in
the pharynx.

Amphioxus feeds on small organisms
and other organic par-
Feedine tides which it filters out

of the water. A current
enters the mouth, passes through the gill slits _ from the pharynx
to the atrium, and leaves by the atnopore ; it is caused to flow by
the action of the cilia on the sides ot the gill bars ( lateral cilia), whu 1
beat outwards. The cilia on the inner faces of the bars ( frontal cilia)
pass the mucus which is secreted by the endostyle upwards across

414 MANUAL OF ELEMENTAL Y ZOOLOGY

the pharynx wall, crossing the slanting clefts, to the epipharyngeal
groove ; the peripharyngeal bands perform a similar function for
the strip of pharyngeal wall in front of the clefts. As the water flows
through the clefts the particles it contains are entangled in the mucus
and carried up to the groove, where they are conveyed backwards
b} ciliary action to the oesophagus. The oral cirri and velar tentacles
strain out coarse and unsuitable matter, and particles which eddy
round under the hood are caught by the wheel organ, which entangles
them with mucus and sends them backward to the pharynx. The
mucus containing the food particles passes through the oesophagus as
a sausage which by ciliary action is carried backward, rotating as it
goes, to the anus. Digestion is performed by the secretions of the

icpatic caecum and gut, and absorption takes place in the intestinal
region.

The resemblance between this procedure and the feeding of the
Microphagy. mussels (p. 376) is very striking. In both water con-
taming the food particles is made to flow through a
meshwork by cilia on the sides of the meshes, the particles are re-
tained upon the surface of the meshwork, and they are transported
thence to the digestive organs by another set of cilia. The habit of
feeding on organic particles (s microphagy ) is ^followed by many
animals. Lsually the particles are gathered by cilia, but amorm
Arthropoda, which have no cilia, those that feed in this way do so by
means of fine bristles. The mosquito larva (p. 346) is an example. '

In the dorsal wall ol the atrium, lying between the
Excretor ytrial epithelium and the dorsal coelomic canals,
Organs. y is a series of short tubular structures, corre-
sponding in number with the primary gill slits
and opening into the atrium opposite the dorsal ends of
the tongue bars. Each passes forwards round the top
ot the adjoining slit and gives off on its upper side several
short branches. From each branch there projects into
the coelom a tuft ot fibres known as solenocytes , which are
fine, protoplasmic tubules, each ending blindly in a knob
which contains a nucleus. The other end of the tubule
opens into one of the branches of the main tube. Each
tubule contains a long flagellum, which arises from the
protoplasm round the nucleus and hangs into the main
duct. These organs are nephridia, although they do not,
like the nephridia of the earthworm, open into the coelom5
Their solenocytes recall the flame cells of the Platv-
helminthes (p. 243). It has been shown that if the animal
be fed with carmine the colouring matter is excreted
through the nephridia. A single nephridium, Hatschek's
nephndium , lies in a coelomic cavity on the left side of the

THE LANCELET

4i5

JU-o

head and opens behind into the pharynx,
funnel-shaped structures, known as brown
lie in the hinder part of
the dorsal coelom and open
into the atrium at their
wide posterior ends, have
also been regarded as ex-
cretory.

The blood is colourless

and contains

Vascular white cor-

System. , ^

puscles. 1 he
vascular system is closed.

There is no heart, but
the larger vessels are
contractile and set up
a circulation. A ventral
Dr branchial aorta in the
subendostylar coelom
gives off to each primary
gill bar a vessel which
divides to take part in the
formation of a rather com-
plicated branchial system
in the primary and sec-
ondary bars and synap-
ticulse. On each side the
branches of this system
are gathered up into a
series of vessels, which
open into a longitudinal
supr abranchial artery (or
lateral dorsal aorta ) beside
the epibranchial groove,
some of the blood passing
through nephridial plex-
uses on the way. Behind
the pharynx the supra-
branchial arteries unite to
form a dorsal aorta, which
runs back under the noto-

A pair of
tubes , which

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4i6

MANUAL OF ELEMENTARY ZOOLOGY

Fig. 314. — The front

part of the nerve
cord of Amphioxus ,
seen from above.

C.C., Central canal ; c.v.,

cerebral vesicle ; pg.,
anterior pigment spot ; pg'.,
pigment spots in the iioor
of the cord ; v.r., ventral
root — the corresponding
dorsal root lies immedi-
ately behind it ; I, II,
first two pairs of nerves.

chord, giving branches to the gut
and body- wall. From these blood
is collected by a subintestinal vein.
I his is for much of its course a
plexus, but in front becomes a
single vessel which runs to the liver
and there breaks up again into a
hepatic plexus. A hepatic vein,
which is joined by a pair of vessels
0 ductus C-uvieri ) from the body-
wall, conveys the contents of this
plexus to the ventral aorta. Com-
parison ot this circulation with
that of the dogfish, presently to be
described, will show that the general
course of the blood is similar in the
two cases. It will be seen that the
direction of flow in the ventral and
dorsal vessels of Amphioxus is
opposite to that in the worm and
the same as that of the fish, that
its gills are supplied in the same
way as in the fish, and that there
is in both a hepatic portal system.

The body contains a number of
lymph spaces. Some of these (as
those in the fins and certain spaces
among the muscles) are of ccelomic
origin. Others, such as the meta-
pleural canals of the adult, may
possibly be haemocoelic.

I he position of the nerve cord
has been described. It

syZmA is rouShly triangular in
Sense Organs, transverse section, being
flattened on its under
side, ends abruptly in front at the
level of the first myomere, and
behind tapers to a point over the
hind end of the notochord. There
is no ventral fissure, but a deep

THE LANCE LET

417

dorsal fissure, which is clearly due to the closure of a tube,
part of which remains as the minute central canal. This
tube is lined by an epithelium, and around it lie nerve
cells, but there are no dorsal and ventral horns. The
remainder of the cord is composed ot non-medullated
fibres. At the anterior end the canal widens out into a
cerebral vesicle , which in the larva communicates by a
pore with a ciliated funnel known as the olf actory pit , on
the dorsal surface of the left side of the body. In the
adult this opening is lost, though the pit remains. Whether
it is sensory is doubtful. A ciliated depression of the
floor of the vesicle perhaps corresponds to the infundi-
bulum of a vertebrate animal. The first two pairs ot
nerves are specialised as cerebral nerves. The first pair
arise from the lower side of the anterior end of the cord,
the second pair from the dorsal surface behind the cerebral
vesicle. These pairs are symmetrical. They are distri-
buted to the epidermis of the snout and are sensory in
function. The remaining nerves are not symmetrical, but
alternate with one another on the two sides, in correlation
with the alternation of the myomeres. Each corresponds
to a dorsal or a ventral root of a spinal nerve 'of .the frog,
the ventral roots being placed in front ot the dorsal. The
roots do not join, the ventral, which are groups of slender
rootlets, passing direct to the muscles, and the dorsal,
which are compact, passing in the septa between the
myomeres to the epidermis. 1 he dorsal roots have no
ganglia, their fibres being in the condition of the afferent
fibres in a worm such as Nereis (Fig. 185, B), that is,
having their nerve cells at the periphery of the body.
They are said also to supply the alimentary canal. The
sense organs are few and simple. Supposed tactile cells
bearing short, stiff processes are scattered among the
ordinary ectoderm cells, especially at the front end of the
body and around the mouth. A mass of pigment which
lies in the front wall of the cerebral vesicle is not sensitive
to light, but small groups of pigmented organs which
occur at intervals on the lower side of the canal in the
cord appear to be so.

The sexes of the lancelet are separate, but show no
differences save in the nature of the gonads. 1 hese are

27

4iS

MANUAL OF ELEMENTARY ZOOLOGY

cubical bodies, twenty-six on each side, placed in the wall
ot the atrium, into which they shed their germs
organs.UCt,ve by rupture of their walls. Each corresponds
to one of the myomeres and consists of a
closed eoelomic sac, whose cavity is known as the gono-
ccele and on whose wall the germs arise, though they are
actually derived, by a rather complicated process, from
the epithelium of the embryonic coelom of the myomere
behind that in which they lie. The egg is minute, but
contains yolk granules. The gametes are carried out by
the current through the atriopore and fertilisation takes
place in the water.

The lancelet is an example of a group of animals known
Chordata. as Chordata, which includes also the backboned
or vertebrate animals and certain less familiar
creatures. Chordate animals are coelomate Metazoa dis-
tinguished by the possession of a notochord, a hollow,
dorsal nerve cord, gill clefts (“ visceral clefts ”), and
almost always a muscular hinder or “ tail ” region which
contains no viscera and is used, either for driving or
steering, in locomotion. In many members of the group,
however, some or all of these features may be present
during development and lost by the adult, 'as the adult
frog has lost the notochord, gill clefts, and tail which
were possessed by the tadpole.

CHAPTER XXI

THE DOGFISH

Various species of the small sharks known as Dogfish are
found in British waters. One of the commonest
of them is the Lesser Spotted Dogfish or Rough
Hound, Scyllium canicula. Like other dogfish, it justifies
its name by travelling in packs and hunting by smell. It
lives usually near the sea bottom, and feeds largely upon
crabs, hermit crabs, and other crustaceans, though it also
often devours shell-fish, or small fishes, and will indeed
take most kinds of animal food, dead or alive. It is very
voracious and is a nuisance to fishermen by taking the
bait meant for its betters. Its flesh, though coarse, is used
for food.

The length of a well-grown rough hound is about two
feet. Its slender, sinister-looking body, well-
Featuree, shaped for passage through the water, tapers
from before backwards, and, though it
shows no sudden differences in size, there may be re-
cognised in it a head, trunk, and tail, the hinder limit of
the former being marked roughly by the hindmost
gill slit (see below), and that of the trunk by the vent. The
head is flat, and has a blunt-pointed snout, a wide,
crescentic mouth on the lower side, a pair of round nostrils
in front of the mouth and connected with it by oro-

nasal grooves, and at the sides two slit-like eyes. Im-
mediately behind each eye is a small, round opening, the
spiracle , while farther back and more towards the ventral
side is a row of slits which are the gill slits or gill clefts. The
spiracle and the gill clefts open internally into the pharynx.
Behind the head the body gradually changes its shape,

becoming flattened from side to side instead of from above

419

420

MANUAL OF ELEMENTARY ZOOLOGY

downwards. The vent or opening of the cloaca lies in a
deep longitudinal groove of the belly,
just before the middle of the body.

Into the same groove there opens at
each side one of the abdominal pores ,
which lead from the body cavity. There
are two pairs of fins and four unpaired
fins. The fore or pectoral Jins, corre-
sponding to the arms of the frog, are a
pair of flat, triangular organs attached
by one angle to the sides of the ventral
surface not far behind the head. The
hinder or pelvic fins are smaller and
narrower structures of somewhat the ;
same shape, attached one on each side
of the middle line of the belly in front
of the vent. In the male, their inner
edges are fused and there projects back-
wards from the under surface of each a
rod, grooved along its inner side, known
as a clasper. The unpaired fins are
median structures in the tail. Two,
known as the anterior and posterior
dorsal fins , are on the back, one, the
ventral fin , is on the under side, and
another, the caudal fin, surrounds the
end of the tail. This fin has two lobes,
and the axis of the tail is turned upwards
and passes into the upper lobe.

Certain generalisations which we have

made in the course of the
G@n«r«i previous chapters enable us

Internal f , , • r j

Feature#. to state in a few words a

good deal of information
about the anatomy of the dogfish. A

Fig. 315. — The Rough Hound.

Note mouth, eye, spiracle, lateral line, gill clefts,
pectoral and pelvic fins, dorsal fins, caudal fin,
ventral fin between caudal and pelvic fins,
f./., Upper lobe of caudal fin ; c.f ., lower lobe of the
same ; pLf-t right pelvic fin.

THE DOGFISH

421

dogfish is a metazoan animal (p. 177)- It is triploblastic
(p. 275). It has a large perivisceral coelom (p. 275) and a
closed circulation (p. 306). It is bilaterally symmetrical
(p. 33)- It is segmented (p. 277), though the primary
segmentation is best seen in the early stages ol develop-
ment and is represented in the adult only by the arrange-
ment of the muscles of the body- wall, the segmentation
which is found in the backbone and nervous system arising

Fig. 316. — Placoid scales.

A A portion of the skin of the rough hound as seen undei a hand lens ; B, a, singl*
scale removed from the skin ; C, the same in section (diagrammatic).

b., Base of the scale ; c., the same in section ; d ., dentine ; e ., enamel ;

p., pulp cavity.

later. It is chordate (p. 418). Lastly, like the frog, it is a
backboned or vertebrate animal. This term implies more
than the possession of a backbone. The V ertebrata are a
large group of animals which have in common, besides the
features we have just mentioned, the following : (1) they
possess an internal skeleton of bone or cartilage, part of
which forms a skull and backbone ; (2) their central nervous
system, which is dorsal and hollow, consists of a spinal
cord and a complicated brain ; (3) the gill clefts, which they
all possess during some period of development, are few and

422 MANUAL OF ELEMENTARY ZOOLOGY

do not open into an atrium ; (4) they have a heart, which
lies below the gut ; (5) most, though not all of them, possess
two pairs of limbs and none has more ; (6) like Amphioxus ,
but unlike certain other C hordata, they are, though in-
completely, segmented.

f pon the back and sides of the rough hound the skin
Skin. *s a grey-brown colour with small spots of

darker brown ; upon the belly it is whitish. It
leels smooth to the hand it it be stroked from head to tail,
but rough if it be stroked in the opposite direction. This
is due to the presence of scales , which are not flat like those

FlG. 317. — The hinder part of the trunk of a dogfish seen from the
left side, with a piece of the skin removed.

"lube of the lateral line; myc., myocommata or septa of connective tissue;

mym., myomeres.

of most fishes, but bear minute spines directed backwards.
Such scales are said to be placoid. Each consists of a
calcified basal plate embedded in the dermis, and a spine
which is composed of dentine covered with enamel/ A
pulp cavity, containing highly vascular connective tissue,
passes through the base into the spine. It will be seen
that the general features of such a scale resemble those of
the tooth of a frog (p. 57). In fact the teeth of the dogfish,
though they are larger, have the structures of the scales’
and we must regard teeth as modified scales.

In the body-wall (p. 35) the muscles are for the most part
segmentally arranged, each muscle-segment being known

THE DOGFISH

423

as a myomere. The myomeres do not lie straight, but
each is bent five times, so that it runs a zigzag
Muscles and course from the middle of the back to that of
Movements. jn the muscles of the head, throat,

and fins the segmental arrangement is not apparent. The
myomeres are separated r
by partitions of con-
nective tissue (myocom-
mata), between which
their fibres run longitu-
dinally. In swimming,
waves of curvature pro-
duced by contraction of
the muscles (especially
powerful in the tail,
which is more than half
the length of the body)
pass alternately down
the two sides. The
leading (backward) face
of each wave presses
upon the water oblique-
ly backwards and to
one side, that of the

next wave presses simi-
larly backwards but to
the other side, and so
their net effect is by
pressing backwards to
drive the fish forwards.

The tail fin (though in
this respect less im-
portant than that of
most fishes) adds to the The crest of a wave is marked by a black dot.
propellant surface which Intervals between the photographs o-io

is applied to the water " .

when a wave reaches the end of the body. In turning,
a strong contraction is sent down one side and turns
the head to that side. The tail, owing to the resistance
offered by its fin, stands firm as a fulcrum for the
head-turn ; afterwards it is swung into line with the

Fig. 318. — Successive positions ot a
dogfish during swimming.— After

Gray.

424 MANUAL OF ELEMENTARY ZOOLOGY

head. As the tail is moved to and fro it is caused by
the larger size of the lower lobe of its fin to rise, and it
would therefore drive the snout downwards were it not
counteracted by the pectoral fins. The function of these,
and to a less extent of the pelvic fins, is, by acting like
the wings of an aeroplane, to raise the forepart of the
body. Thus the whole body is held up in the water without
the air bladder which some fishes possess. The dorsal and
ventral fins act like the keel of a boat in keeping the body
upright.

The scales of the dogfish are a part of the skeleton
which, being on the surface of the body, is
Genera" : known as the exoskeleton , and in the frog is

features. represented only by the teeth. The endo-
skeleton of the dogfish corresponds to that of
the frog in its main outlines, but differs from it in some
important respects, (i) It is wholly cartilaginous, like
that of the tadpole, containing nothing which corre-
sponds either to the membrane bones or to the cartilage
bones of the frog, though in places the cartilage is
calcified. (2) The axial skeleton (p. 36) is traversed
longitudinally below the central nervous system by a
peculiar rod, the notochord, which consists of large
vacuolated cells with stout walls, and is derived, in
the course of development, from the roof of the
primitive alimentary canal. A notochord is present
in the tadpole, but in the adult frog is represented
only by pads of tissue between the centra of the
vertebrae. (3) There are no structures which represent
any part of the sternum. (4) In correspondence with
the difference in the form of the limbs, their skeleton
differs entirely in the two animals. (5) Unlike that
of the tadpole, the median fins are supported by
rays.

The backbone consists of about 130 vertebrae (p.
Backbone 3 6), in each of which the centrum is pierced
from end to end by a canal for the noto-
chord. This canal is narrower in the middle of the
vertebra, so that the'*noto'chord is constricted, and after
its removal the centrum appears as a biconcave disc.
On each side the centrum bears a pair of ventrilateral

V. VI. VII. oi). VII.

THE DOGFISH

425

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426

MANUAL OF ELEMENTARY ZOOLOGY

processes} In the trunk region these are directed out-
wards and bear short ribs , which lie beneath the muscles
of the back ; in the hinder part of the body the pro-
cesses are directed downwards and are known as hcemal
arches , enclosing a hcemal canal , in which lie the
caudal artery and vein. Towards the hinder end of
the tail they fuse at their ends and bear a median
hcemal spine . Between the neural arches of successive
vertebrae are wide gaps which are closed by intercalary
pieces. A series of flat median pieces of cartilage, the
supradorsals , twice as numerous as the vertebrae, fill
the gaps between the tops of the neural arches and
intercalary pieces and roof in the vertebral canal.

The skull consists, like that of the frog, of a cranium
Skuti which contains the brain, with a pair of nasal

capsules in front, a pair of auditory capsules
one at each side of the hinder end, and a visceral skeleton
below. The nasal capsules are large, thin-walled struc-
tures, continuous with the cranium, widely open below,
and separated by the cartilaginous internasal septum or
mesethmoid cartilage. Three slender processes, one from
the front wail of each capsule and one from the mes-
ethmoid cartilage, project into the snout and are together
known as the rostrum. At the junction of the cranium
and the nasal capsules the roof of the skull shows a
large gap, the anterior fontanelle. From the sides of
the cranium large supraorbital and suborbital ridges
project above and below the orbit. On the auditory
capsules, which are continuous with the cranium, ridges
mark the position of the semicircular canals. A pit
on the roof between the auditory capsules receives on
each side a canal, in which a tube, the aquceductus
vestibuli (the ductus endolymphaticus, p. 101), runs from
the ear labyrinth to a small opening above, by which
the endolvmph communicates with the sea water. There is
no ear drum. At the hinder end, between two occipital con-
dyles, is seen the notochord, which passes into the floor of the
cranium for some distance. Numerous openings pierce the

1 These are often called transverse processes, but they do not corre-
spond with the transverse processes of the frog, which belong to the
neural arches.

THE DOGFISH

427

r ost

°P-

a.Vr >

nch. o.c.

Fig. 320. — Parts of the skeleton of the dogfish.

A , The skull, from above ; B, the same, from below ; C, skeleton of
visceral arches, not including the labial or extrabranchial cartilages;
D, section of a trunk vertebra ; E, section of a tail vertebra.
a.v., Opening of tube from inner ear ; b.b., b.h., cer.b., cer.h., e.c.f.,
gr., hymd., M.C., n.a., nas.c., orb., op'., pal.b., ph.b., rost., spd.,
tr., as in Fig. 300; car. g., groove for carotid artery ; e.c.f., foramen
for orbital (“ external carotid ”) artery ; f.m., foramen magnum ;
font., fontanelle ; h.b., hypobranchial cartilage ; hae.c., haemal

canal ; hy,.f., facet for hyomandibular cartilage ; i.c.f., foramen
for internal carotid arteries; m.e., mesethmoid cartilage; nch ,
notochord ; o.c., occipital condyles; sem.c., semicircular canals-
ttr.c., vertebral foramen.

428

MANUAL OF ELEMENTARY ZOO LOO Y

sivam*

IV

V mat

V md. -
op. -
VII pal.
VII hm.

VJfptsp. '
VJIern. ,
VII im.
VII hd'
VIII %
IX''

X g.s.'

Xv.--

XI.---

Sp.7lr -

- a.c.s .

-olf.l.

o.s.

eye
- r.int .
r.s.
r.e.

■a mp.
-h.s,
-a. us.
utr.
-p. v.s.

Fig. 321. — A dissection of the nervous system and sense organs

of a dogfish.

On the left : nerves labelled as in Figs. 336-340.

On the right : a.c.s., anterior cardinal sinus ; a.v.s., amp., h.s., p.v.s., utr., parts
of labyrinth, labelled as in Fig. 55 ; au.c., auditory capsule ; cer., olf.l., 0., ol., L,
sp.c., as in Fig. 336 ; n.am., neuromast ampullas ; os., r.e., r.int., r.s., s.p., as
in Fig. 339.

The eye and part of the auditory capsule, which have been removed from the left-
hand side, remain in situ on the right. The cartilage of the skull and vertebras
is dotted, and the nerves are seen passing through the foramina shown in
Fig. 319.

THE DOGFISH

429

wall of the cranium. (1) On the roof lies the anterior fontan-
elle which we have already mentioned. (2) At the front end
two large foramina put the cranial cavity in continuity with
those of the nasal capsules. Through these the olfactory
nerves pass from the surface of each olfactory lobe of the
brain into the olfactory organ. (3) On each side wall num-
erous openings allow the passage of nerves and blood vessels
to and from the orbit. The relative sizes and positions of
these are seen in Fig. 319. (4) Just behind the auditory

capsules, at the bottom of a deep pit, is the foramen for the
ninth nerve, and on each side of the occipital condyles is a
foramen for the passage of the tenth nerve. (5) On the under
side there may be seen two shallow grooves, along which
the internal carotid arteries run. Where these meet there is
a small opening, through which the two arteries enter the
cranium. At the outer ends of the grooves are the openings
through which the orbital (so-called external carotid) arteries
pass from the roof of the mouth to the orbits. (6) At the
hinder end of the skull is the large foramen magnum. The
visceral skeleton is a series of seven arches (p. 436), each con-
sisting of several pieces, which lie at the sides of the mouth.
The first of these is the mandibular arch , which forms the
skeleton of the j aws . Each upper jaw -bar or palato-ptery go-
quadrate cartilage is a rod which meets its fellow in front
of the mouth and is there joined to it by a ligament. It is
attached to the cranium in front of the orbit by the ethmo-
palatine ligament and behind to the auditory capsule by a
postspiracular ligament. Each half of the lower jaw is formed
by Meckel’s cartilage , which is a wide, flat bar, tapering
forwards to a point, where it is joined with its fellow by a
ligament. It articulates behind with the palato-pterygo-
quadrate cartilage. Both the upper and the lower jaw bars
are joined by ligament to the hyomandibular cartilage which
forms their principal attachment to the skull. The second
or hyoid arch consists of two pieces, the hyomandibular
cartilage , which is a short, stout rod articulated with a large
facet on the side of the auditory capsule, and a longer,
more slender, ceratohyal cartilage , which passes forwards
and inwards from under the hyomandibular to join a
median plate, the basihyal cartilage , in the floor of the mouth.
The remaining five arches are the branchial arches.

430

MANUAL OF ELEMENTARY ZOOLOGY

Each branchial arch contains above a flat, pointed p ha ryngob ranch ial
which, starting beside the backbone, slopes forwards to join an epi-
branchial that lies at the side of the pharynx in a line with the
hyoniandibular cartilage. From the lower end of this the ccrato-
branchial runs forwards and inwards parallel with the ceratohyal and
mandibular cartilages. The first four ceratobranchials are connected
with hypobranchials in the floor of the pharynx. The first hypo-
branchial is small and joins the first ceratobranchial with the baiihyai ;

coy., Coracoid region ; dors., dorsal end of scapula ; gl., glenoid facet ; h.r., horny rays
(dermotrichia) ; except on the outer border of the fin shown these have been cut
away where they covered the cartilaginous rays ; mpl., metapterygium ; ms pi.
mesopterygium ; ppt., propterygium ; rad., cartilaginous rays ; sc., scapula.

the three hinder are larger and directed backwards and inwards. The
last two pairs of hypobranchials and the fifth ceratobranchials join a
median basikranchial plate. The epibranchial, ceratobranchial, hyo-
mandibular, and ceratohyal cartilages bear gill rays along their hinder
borders. Outside the upper and lower jaws lie a pair of labial
cartilages , and along the outer sides of the second, third, and fourth
ceratobranchials are extrabranchials.

The median fins are supported by a skeleton consisting ot several
series of rays. The series nearest the body are cartilaginous rods known

THE DOGFISH

43i

as basalia and are attached to the neural and haemal spines. They are
succeeded by a similar series known as radialia , and these by two rows
of small, polygonal plates of cartilage which are overlapped at the
sides by a double series of horny rays or dermotrichia that project
beyond them. The dermotrichia, which belong to the dermis, are
tine fibres composed of the same substance ( elastin ) as the elastic
fibres of connective tissue. In the caudal fin the cartilaginous rays
are not distinct from the supradorsals and haemal spines.

Limbs.

The limbs are anchored into the body by girdles which
corre- .

, I sc.pu. ac.

\ / ,il.

r.

spond
to those of the
frog. The pec-
toral girdle con-
sists of two
curved pieces of
cartilage, at the
sides of the body,
of which the
lower ends are
fused in the mid-
ventral line. To
the hinder sides
of these pieces
are articulated
the fins. . The
surface of articu-
lation is the
glenoid facet ,
the portion of
the girdle above
the facet being
the s cafular
region and that
below, the cora-
coid. The scapula is rod-like ; the coracoid is broad and flat
and supports the floor of the pericardium. The pectoral fin
articulates with its girdle by three basal cartilages, the pro-,
me so-, and metapterygia , of which the former is the anterior
and smallest, the metapterygium the hindmost and largest.
Along the outer borders of these pieces are set a series of
radialia. The pro- and mesopterygia each bear one stout

Fig

ac.,

323. — The skeleton of the pelvic fins and
girdle of a female dogfish

Acetabular surface ; bp ., basipterygium ; k.r., horny
rays; it., iliac process; isc.pu ., ischio-pubic region;
rad., cartilaginous rays.

432

MANUAL OF ELEMENTARY ZOOLOGY

ray, the metaptery-gium several, which are slender. To the
end of these, smaller, polygonal pieces are attached, and
these in turn bear a double series of horny dermotrichia
which overlap them above and below and project beyond
them. The pelvic girdle is a stout, straight bar of cartilage,
placed athwart the belly and bearing a blunt knob at each
end. The main part of the bar is the ischiopubic region ,
the knobs are the iliac processes . and the fins articulate
with an acetabular facet upon the hinder border at the base
of the iliac processes. The fin has a long inwardly-curved
basipterygium , bearing a row of radialia along its outer side.
In the male it also bears a long piece of cartilage which
supports the clasper.

Fig. 324. — The hinder wall of a gill pouch of the dogfish.

br.a., Branchial arch, flexed because the pharynx floor is raised ; eb., eb.',
extrabranchials ; g., gill ; hyp., hypobranchials in section ; m.e.o., margin
of external opening.

The perivisceral cavity (coelom) is divided into two parts,
the small pericardial cavity just in front of the
AHmentary pectoral fins, and the large peritoneal cavity

System. behind it, between the two pairs of fins. The

two cavities are divided by a membranous
septum, but a narrow passage, the pericardio-peritoneal
canal , leads from one to the other below the oesophagus.
From the peritoneal cavity the two small abdominal pores
lead to the exterior, one on either side of the vent. This
cavity contains, like the pleuro-peritoneal cavity of the
frog, among other organs, the whole of the alimentary
canal with the exception of the mouth and pharynx. The
gape of the mouth is edged with several rows of teeth,

THE DOGFISH

433

which, as we have seen, are simply enlarged scales. These
lie in a part of the skin which passes over the jaw and is
tucked into a groove within it. They are not in any way

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434

MANUAL OF ELEMENTARY ZOOLOGY

THE DOGFISH

435

attached to the jaw. As they wear away they are replaced
by new rows which are constantly being formed in the
groove and carried up over the edge of the jaw by the
growth of the skin. Like those of the trog their function
is only to hold the food, which is swallowed whole. The
pharynx is only distinguished from the mouth cavity by
possessing the inner openings of the spiracle and gill clefts
These are placed between the arches of the visceral
skeleton, the first gill cleft lying between the hyoid and
first branchial arches. The clefts do not pass straight
outwards through the wall of the throat, but the outer
opening of each is at some distance behind the inner, so
that the cleft is a pouch which slants backwards and out-
wards from the pharynx to the exterior. The pouches are
spacious cavities, being deep, and considerably taller than
their openings at either end, though the inner opening is
larger than the outer. On each wall of the pouch lie a
number of folds which constitute a gill. These are highly
vascular, and in fresh specimens have consequently a
bright red colour. 1 here is a gill on each side of each
cleft except the last, which has no gill on the hinder side.
The spiracle is a small cleft of the same series as the gill
cleft, and bears on its trout side a vestige of a gill, known
as a pseudobranch. The regions between the clefts, and
those immediately in front of the first cleft (the spiracle)

Fig. j>26. A female dogfish in which the abdominal and pericardia]
cavities have been opened from the ventral side, and the viscera
somewhat displaced. The pericardium has been opened slightlv
to the left of the middle line, and the right lobe of the liver ha's
been cut away.

a r r ?°a6Si ’ b'' b!je k UCt ’ c‘! cardiac ]imb of stomach ; c.ar., caudal
artery , c.v., caudal vein ; d., bursa entiana ; g.b., portion of gall bladder appear-
mg on surface of left lobe of liver in which it is embedded ; i.f intestine; ia,m-
chord3-1 iani° antL10r mesenteric artery ; lienogastric artery ; not., notc-
J ,rd T* y PC> PortaI vein Jymg beside hepatic artery ; ps., pancreas
he t! in°HeninV ? intestine; py., pyloric limb of stomach f r., rectum,
f ends of oviducts with rectal gland (r.gl.) attached to its dorsal

r St Jill TP i 0ry fgamenft °f llver> wjth internal opening of oviducts ; sh..
right shell gland on course of right oviduct ; sp„ spleen ; sp.c., spinal cord

antf splenicrveins>llla ’ V’’ brancb of portal vein formed by junction of intestinal

Besides the above, note— -nostrils ; oronasal grooves ; mouth ; pectoral and pelvic

?hihinH\enCar??1i and abid0JT‘al- cavities ; heart, consisting of sinus versus
( ehind), ventricle, auricle (showing at sides of ventricle), and conus ; cloaca
and transverse section of tail, showing at the sides the myomeres, above the
anterior dorsal fin and in the middle the cartilage of the backbone enclosing
spinal cord, notochord, and blood vessels. "*

436 MANUAL OF ELEMENTARY ZOOLOGY

and behind the last, are known as visceral arches , and
named, in order, mandibular, hyoid, and first to fifth
branchial. Each contains a skeletal arch, arteries, and a
nerve (Fig. 345). The spiracle separates the mandibular
and hyoid arches. The movements of the visceral arches,
which carry out the processes of feeding and, as we shall
see, of breathing, are brought about by muscles which
run between the cartilages of the arches and the coracoid
region of the shoulder •, girdle. From the pharynx the
narrower oesophagus leads back through the coelom to the
stomach. This is sharply divided into a cardiac and a
pyloric part. The former is a sac, in shape not unlike the
stomach of the frog ; neat its hinder end on the right side
arises the narrow tubular pyloric division, which runs
forwards beside the cardiac. At its: front end a slight
constriction marks the presence of the pyloric sphincter
and divides it from the intestine. The main part of the
intestine is the ileum, a long, wide sac which passes
backwards towards the cloaca and has its internal surface
increased by a spiral fold of the mucous membrane known
as the spiral valve. Between this region and the pyloric
sphincter lies a short, somewhat narrower region called
the duodenum or bursa entiana , which is without a spiral
valve and receives the ducts ot the liver and pancreas. At
its hinder end the ileum narrows and loses its spiral valve,
thus becoming the rectum, this in turn ending in the wider
cloaca, which receives; the urinary and generative ducts
and opens by the vent. There is no bladder. The liver is
a very large organ, consisting of long right and left lobes
united in front and slung by the suspensory ligament from
the anterior wall of the peritoneal cavity. The gall bladder
is embedded in the front part of the left lobe ot the liver,
but usually a part of it show$ upon the surface. From it
the bile duct runs backwards to open into the intestine,
lying in the membrane or omentum which carries the
hepatic artery and portal vein. The pancreas lies between
the stomach and intestine ; it is long and narrow and has
in front a rounded ventral lobe, from which its duct passes
to the ventral side of the intestine. The rectal gland is a
small, cylindrical structure which opens into the dorsal
side of the rectum by a duct.

THE DOGFISH

437

By a munching action of the floor of the mouth and
pharynx the fish incessantly renews the water
Respiration. that pathes its organs of respiration, the gills.

When the floor is lowered water is drawn in through the
mouth, while the flexible front edge of each gill cleft is
caused, by the lower pressure within, to flap back so as to
prevent the entry of water that way. When the floor is
raised the lips prevent the escape of water through the
mouth, contraction of the oesophagus keeps that also closed,
and the water, being under pressure, opens the clefts and
passes out over the gills. The shape
of the gill pouches must cause the
water to eddy about the gills as it
goes. Through the thin membrane
which is all that separates the
blood in the gills from the water,
the gases of respiration are ex-
changed.

The spleen must be mentioned
here, although it has no connection
with the alimentary canal. It is
attached by membrane to the hinder
end of the stomach as a triangular
lobe with a forward prolongation
along the right side of the pyloric
division.

The kidneys of the dogfish, like those of the trog
(p. 78), lie above the abdominal cavity, out-
Excretory and side the peritoneum, and are masses of tubules.
nrran«t,ve In the embryo the tubules correspond with
the muscle segments, but later more are
added to these primary .ones. Unlike those of the frog,
many of the primary tubules keep a minute peritoneal
funnel , opening to the coelom, which they had in the
embryo. The kidneys are relatively larger than the frog s,
and each consists of a wide part behind, which lorms a
cushion-like swelling and is the principal excretory organ,
and a narrow part in front, which in the female is a meie
vestige, only to be found by removing the peritoneum, but
in the male is larger, and, with the Wolffian duct coiled upon
it, makes a ridge along the body cavity. The Wolffian duel

Fig. 327.— Diagram of
spiral valve. — After
T J. Parker.

438 MANUAL OF ELEMENTARY ZOOLOGY

1.

runs the whole
length of the
kidney, lying
upon its ven-
tral face and
receiving the
tubules of the
narrow anterior
part. In the
female it is
straight and its
hinder end is
widened to form
a urinary sinus ,
which joins its
fellow to open
into the cloaca
upon a median
urinary papilla \
in the male it is
coiled and serves,
as in the frog,
for the vas def-
erens, its swollen
hinder part being
a vesicula semin-
alis. The tub-
ules of the hinder
part of the kid-
ney join five
of six ducts,
which in the
female open into
the urinary sinus
of their side but

Fig. 328. — The reproductive organs of a female in the male unite

dogfish. to form a so-

cLf Cloaca ; i.o.d., internal opening of the oviducts ; k., k'.,
anterior and posterior parts of right kidney ; msov .,
mesovarium ; o.d., oviduct ; oes., oesophagus ; ov.,
ovary ; rm., rectum ; s.l., part of the suspensory
ligament of the liver ; sh., shell gland ; ur'., ducts of
posterior part of kidney ; u.p., urinary papilla ; ur.s.,
urinary sinus ; IV. d., Wolffian duct.

called ureter ,
which passes
backwards to
open separately

THE DOGFISH

439

from the vesicula semin-
alis into a median urino-
genital sinus. This
has two forward horns
known as the sperm
sacs, which lie upon
the ventral faces of the
vesiculse seminales,
and opens behind
into the cloaca by a
urinogenital papilla be-
hind the anus. The
differentiation, in the
adult kidney, of a hinder
urine-forming part,
with its own duct,
from an anterior part,
vestigial in the female
and sperm-conducting
in the male, is carried
further in higher ani-
mals — reptiles, birds,
and mammals (as the
rabbit, p. 548), where
two such parts are
separated, the hinder
or actual kidney being
called the metanephros
and the anterior the
mesonephros.1

1 The terms “ meso- ”
and “ metanephros ” sig-
nify “ mid- ” and “ hind-
kidney,” and the reason
for which they are applied
to the parts of the kidney
which bear them is that in
the embryo of every verte-
brate (though very rarely
in the adult) there lies in
front of these parts a region
known as the pronephros.
or “ fore kidney.” That

s- 1.

cl., Cloaca ; i.o.d., rudiment of internal opening
of oviducts ; l.t., left testis ; k., k' an-
terior and posterior parts of left kidney ;
mso., mesorchium ; oes., oesophagus ; r.t.,
right testis ; run., rectum ; s.l., suspensory
ligament; sp.s., left sperm sac; u.g.p.,
urinogenital papilla ; u.g.s., urinogenital
sinus ; ur., ureter ; ur\, ducts of posterior
part of kidney ; v.eff., vasa efferentia ; ves.
sem., vesicula seminalis ; IV. d., Wolffian
duct or vas deferens.

440

MANUAL OF ELEMENTARY ZOOLOGY

The principal end-product of nitrogenous metabolism is urea.
Much of this, however, is retained in the blood (see p. 448).

There is a single ovary, which probably represents that
of the right side of the frog. It hangs into the body cavity
and varies in size and appearance with age. The ova are

in different stages of ripeness,
the ripest being very large
and yolky. They are shed
into the body cavity and
passed forwards by con-
tractions of the abdominal
walls to the front of the
peritoneal space where they
enter the internal opening
of the oviducts . The latter
are large straight tubes, one
on each side of the body,
attached to the dorsal wall
of the coelom. They start,
from a common opening in
the suspensory ligament, not
far behind which each has
a round swelling known as
the shell gland, by which the
shells of the eggs are secreted.

Fig. 330. — An embryo dogfish ^ s ^1( hinder end of the
in its egg-case (“ mermaid’s trunk they enter the cloaca
purse ”) which has been cut by a common opening just
* ®Pen to show the contents.— behind the anus. The testes

, n . , n . „ are a pair ot long organs

gills; st.t stalk of yolk-sac; t., ten- shmg by membranes from
mtamPnrf0lS10f • of eggTase b? the dorsal wall of the coelom.

means of which it is moored to sea- T. . . .

weed ; y.s.« voik-sac. .bach communicates at its

front end with the kidney of
its side by several small vasa efferentia, the sperm passing
through these into kidney tubules and thence to the vas
deferens or Wolffian duct, by which it is conveyed to the

part of the original kidney which lies behind the pronephros and in
some animals is separated into meso- and metanephros receives,
as a whole, the name opisthonephros , or “after kidney.” In the
dogfish, and in the frog, the adult kidney is the opisthonephros.

THE DOGFISH

44‘

Fig. 331. — The forepart of the body of a dogfish, dissected to show
the heart and ventral arterial system.

t.b.s.. Afferent branchial arteries; aw., auricle ; c.a., conus arteriosus ; ch., cera-
tohyal cartilage ; d.C., ductus Cuvieri ; g., gills ; g.c., gill clefts ; i.o., internal
opening of the first gill cleft ; Is., line of section in Fig. 325, which should be
compared ; M.c., Meckel’s cartilage ; mu., muscles from coracoid region of
shoulder girdle to various parts of visceral skeleton ; p.m., pericardium ; s.v.,
sinus venosus ; sc., scapula ; thy., thyroid gland (displaced) ; v., ventricle ;
v.ao., ventral aorta.

442

MANUAL OF ELEMENTARY ZOOLOGY

urinogenital sinus. A rudiment of the internal opening
of the oviducts is found in the suspensory ligament of
the male. Sperm is passed by the aid of the claspers
into the cloaca of the female and fertilisation takes place
within her.

It is possible that the sperm is washed out of the grooves of the
claspers by sea water injected into them by the “ siphon ” — a muscular
sac which lies under the skin of the ventral surface in the pelvic region
and communicates by two channels with the grooves.

The eggs are laid in flat, oblong, brown shells whose
angles are prolonged into tapering tendrils, which twine
round seaweeds and thus anchor the egg. Protected by
the shell, the young dogfish develops slowly at the ex-
pense ol the yolk, which comes to be contained in a sac
attached to its belly. At one stage long, vascular threads
project from the gill clefts of the little fish. These are the
so-called external gills, but they are covered with endo-
derm and thus differ from the true external gills of the
tadpole.

The heart of a dogfish lies in the pericardium between
the hinder gill-clefts of the right and left sides.
He°a0rtVesse,S ' ^ a median structure with muscular walls,
and consists essentially of an irregular tube,
bent twice like an S (Fig. 325) and composed of four
successive chambers. The hindermost chamber is the
thin-walled sinus venosus, which is triangular as seen
irom below, and lies with its base against the hinder wall
of the pericardium. In front of it comes the thicker walled
auricle or atrium. This is also triangular, with its apex
forwards, and has its hinder angles widened into pouches,
but is not divided into two chambers like that of the frog.
The S then curves downwards, as the very thick-walled,
conical ventricle, which lies below and somewhat behind
the auricle. From it the narrow conus arteriosus passes
forwards through the front wall of the pericardium to
become the ventral aorta, which is merely the foremost part
of the single vessel whose thickening and twisting produces
the heart behind. Thus the heart, or contractile blood-
vessel, of the dogfish, like that of the frog and all other
vertebrate animals, is ventral in position, whereas the
principal contractile vessel of an invertebrate is generally

THE DOGFISH

443

a.b.a., Afferent branchial arteries ; coe.a., coeliac artery ; d.ao., dorsal aorta ; e.b.a.,
efferent branchial arteries ; en., nostril ; epibr., epibranchial artery ; h.m ., hyo-
mandibular cartilage ; i.c., internal carotid foramen ; inf., infundibulum ; M.c.,
Meckel’s cartilage in lower jaw ; o.i., inferior oblique muscle ; o.s., superior
oblique muscle ; olf.o., olfactory organ ; orb., orbital or “ external carotid ” ;
p.d.a., prolongation of aorta ; r.c.a., carotid root ; sc., scapula ; scL, subclavian
artery ; sk., skull ; sp., spiracle ; sp.a., spiracular artery ; V.md., V.mx.,
mandibular and maxillary branches of fifth nerve ; II., optic nerve.

444

MANUAL OF ELEMENTARY ZOOLOGY

dorsal. The heart contracts from behind forwards, and
drives blood into the ventral aorta, reflux being prevented
by valves at the opening of the sinus into the auricle and
again at the auriclo- ventricular opening, and by two rings
of semilunar or watch-pocket valves in the conus.

The ventral aorta lies in the middle of the throat, below
Arteries. the Pharynx and between the gill clefts, giving
off afferent branchial arteries to the fourth,
third, and second branchial arches, and ending by dividing
into two vessels, each of which again forks to supply the
first branchial and hyoid arches of its side. There are thus
five afferent branchial arteries. These, together with the
ventral aorta, form the ventral arterial system . The
thyroid gland, an organ of internal secretion (p. 62) which
does not belong to the vascular system, lies below the
anterior end of the ventral aorta as a pear-shaped body
with the stalk forwards. From the afferent branchial
arteries the blood passes into the capillaries of the gills,
where it is oxygenated and gathered up into efferent
branchial arteries. These form a complete loop round
each of the first four clefts, the loops being joined fore and
aft by short horizontal vessels at about the middle of their
lengths. The last cleft, having no gill on its hinder side,
has an efferent vessel on its front side only, and all the
blood of this vessel passes by the horizontal vessel into
that of the gill in front. From the dorsal end of each of
the complete loops arises a vessel known as an epibranchial
artery , which runs backwards and inwards on the roof
of the pharynx to join the median dorsal aorta opposite
to its fellow of the other side. From the dorsal end of the
first efferent branchial artery, just outside the origin of
the first epibranchial artery, arises the root of the carotid
artery} This runs forwards and inwards under the skull
and is presently joined by a small branch from the dorsal
aorta (see below), after which it becomes the carotid
(internal carotid) artery. Behind the orbit it gives off
forward an orbital branch which immediately passes
through the opening we have mentioned (p. 429) and
runs forwards along the floor of the orbit to supply the

1 This is often called the “ common carotid artery,” but it does not
correspond to the vessel of that name in the frog.

Inn a.mes coeln d.an epibr. pda.

445

446

MANUAL OF ELEMENTARY ZOOLOGY

upper jaw and the snout. This branch is often called
the external carotid artery but does not correspond to the
external carotid (lingual) of the frog. The carotid artery

co.ullJll,c) Its course in the carotid groove, towards the
middle line, where it unites with its fellow for a short
distance but separates again, passing through the internal
carotKl foramen into the cranium to supply the brain.
Outside the carotid root yet another artery arises from the
first efferent branchial vessel. This is the sfiracular arterv
w hich starts m a line with the horizontal vessels that join
the loops runs forwards to the spiracle, where it supplies
the pseudobranch, crosses the orbital floor, enters the
cranium by a small foramen in the inner wall of the orbit
and .loins the internal carotid artery. The dorsal aorta
ends m front by breaking into two small prolongations
tat curve outwards and join the carotid roots, forming
the definitive carotid arteries. Just before the dorsal
aorta is joined by the last pair of epibranchial vessels it
gives off a pair ot subclavian arteries, which pass back-
wards and outwards to the fore-fins. Behind the pharynx
it runs backwards along the whole length of the body
below the backbone, lying, in the tail, in the htemal canal
as Vast caudal artery. Besides paired vessels to the bodv-
wall. it gives off to the viscera several median vessels
known successively as the cceliac (of which the hepatic
is a branch), anterior mesenteric (of which the genital is a
branch), henogastric, and posterior mesenteric, and to the
kidneys several paired renal arteries.

The sinus venosus receives the whole of the blood re-
Veins. turning to the heart by a number of very lar<re

, . 'tins which are called sinuses , though, unlike

the sinuses ot the cockroach, they do not take the place of
capillaries as well as veins in the circulation, but are merelv
enlarged parts of the veins. The blood from the liver
returns direct to the sinus venosus by two hepatic sinuses
which enter its hinder side. The rest of the blood is
returned by two large precavals or ductus Cuvieri which
join the sinus venosus, one on each side in the pericardium
Into these the blood from the region of the body in front of
the fore-fins is conveyed by a pair of large dorsal anterior
cardinal sinuses and two smaller external or inferior

Fig. 334. — A diagram of the venous system of the dogfish.

a.c.s., Anterior cardinal (or internal jugular) sinus ; bra.s., brachial sinus ; c.v.,
caudal vein ; d.C., ductus Cuvieri, or precaval sinus ; d.l.s., deep lateral
(abdominal) sinus; e.j.s., external or inferior jugular sinus ; h.p.v., hepatic
portal vein ; h.s., hepatic sinus ; hy.s., hyoidean sinus ; i.o.s interorbital sinus ;
il.s., iliac sinus ; int., intestine ; k., kidney ; lr., liver ; n.s., nasal sinus ; or j.,
orbital sinus ; p.c.s., posterior cardinal sinus ; r.p.v., renal portal vein ; s.v.„
sinus venosus ; scl.s., subclavian sinus ; sub.c.s., subscapular sinus ; sup.l.s .,
superficial lateral sinus ; fr.v., transverse vessel joining subclavian sinuses.

447

448 MANUAL OF ELEMENTARY ZOOLOGY

iugular sinuses , below the throat. Each anterior cardinal
sinus communicates in front with an orbital sinus around
the eye, and this in turn with a nasal sinus around the olfac-
tory organ. A hyoidean sinus in the hyoid arch joins the
anterior cardinal and inferior jugular. At the outer end of
each ductus Cuvieri a subscapular sinus enters from the fore-
fin. On its hinder side a very large posterior cardinal sinus
brings back blood from the trunk. The two posterior
cardinal sinuses converge backwards, growing narrower,
and lie side by side between the kidneys, from which blood
passes into them by numerous renal veins. On each flank
two lateral sinuses return blood from the body-wall, and
into one of these open vessels from the fins. Blood from
the tail is returned by the caudal vein ; this divides opposite
the hinder ends of the kidneys into two renal portal veins,
which run forwards along the outer sides of the kidneys and
supply them with blood. Blood from the alimentary canal
and spleen is conveyed to the liver by a hepatic portal vein,
and thence, after passing through capillaries, is discharged
into the hepatic sinuses. It will be noticed that the circula-
tion of the dogfish (Fig. 335) makes a single circuit only,
the blood oxygenated in the respiratory organs being
carried directly to the rest of the body without first return-
ing to the heart as in the frog (p. 76) : the pressure of this
blood is therefore low. The blood cells of the dogfish
resemble those of the frog. The plasma is extremely rich
in urea.

By that fact hangs a surprising and interesting tale. The solids in
. the Blood of most fishes give it a higher osmotic pressure

Regulation than that of fresh waters and lower than that of sea
water. Freshwater fishes are therefore liable to gain
by osmosis from their surroundings more water than is good for them
(p. 79). They put the matter right by forming a very dilute urine :
the filtrate formed in the glomeruli is rendered more watery by the
abstraction of solids from it by the kidney tubules. Marine fishes are
liable to the opposite danger of the loss of water. Bony fishes counter
this by drinking sea water and excreting its salts through the gills
and kidneys, so that there is a net gain of water. But the dogfish and
other cartilaginous fishes have adopted the remarkable expedient of
retaining much of the urea which results from their nitrogenous
metabolism, and so raising the osmotic pressure of their blood till
it is slightly above that of sea water. As a result, a moderate amount
of water enters their bodies but this is got rid of through the kidneys

THE DOGFISH

449

as in the freshwater fishes. The retention of urea is brought about
by a special section of each kidney tubule, which re-absorbs it from
the filtrate that is formed by the glomeruli.

The spinal cord of the dogfish resembles in most respects
that of the frog (p. 84) and need not be described
Central here. The brain, although in general features

System, it is like that of the frog, shows considerable

differences in detail. The foremost region in
the middle line is the cerebrum , which corresponds to the

Sinus vencsus
Auricle
Ventricle

Conus arteriosus
Ventral arterial system
G fills

Dorsal arterial system

cerebral hemispheres of the frog, but is single and some-
what globular in shape ; its double nature is shown out-
wardly by a shallow longitudinal groove and internally by
the presence of two lateral ventricles. It has no cortex.
The two olfactory lobes lie at the sides of the cerebrum,
each arising from it by a short, stout stalk, which expands
29

450

MANUAL OF ELEMENTARY ZOOLOGY

into a large mass against the olfactory capsule. The
lateral ventricles of the cerebrum are continued into the
olfactory lobes. The cerebrum is followed by a long

Fig. 336. — The brain of the dogfish, seen from above.

cb., Cerebellum; cer., cerebrum; m.o., medulla oblongata; olf.l,, olfactory lobe;
ol/.o., olfactory organ ; op. , ophthalmic branches of fifth and seventh nerves;
ep.l,, optic lobes; p.st., pineal stalk ; r.b., restiform body; sp.c., spinal cord ;

spinal nerve ; thal thalamencephalon ; 3, 4, third and fourth ventricles ;
II.-V., V//.-X., cranial nerves.

THE DOGFISH

45 1

thalamencephalon, containing a spacious third ventricle
from the hinder part of whose thin roof arises the long,
hollow, slender pineal stalk, which runs forward over the

Fig. 337. -The brain of a dogfish, in ventral view.

ter., Cerebrum ; inf., return limb of infundibulum, constituting the ventral lobe of
the pituitary body ; l.i., lobi inferiores ; m.o., medulla oblongata ; n.i.l., neuro,
intermediate lobe of pituitary body; olf.l., olfactory lobe; olf.o., olfactory
organ ; op., ophthalmic branches of fifth and seventh nerves ; sp.c., spinal
cord ; s.v., saccus vasculosus ; II.-X., cranial nerves.

cerebrum to end in a small swelling below the membrane
which covers the anterior fontanelle. On the floor of the
thalamencephalon, the infundibulum bears at the sides a
pair of thick-walled lobi mferiores and behind these a

45 2 MANUAL OF ELEMENTARY ZOOLOGY

thin -walled, vascular expansion known as the saccus
t'ascu/osus, alter which it expands as th e posterior {neuro-
intermediate) lobe of the pituitary body (p. 64) and then
turns forward as the ventral lobe of that structure. The
anterior lobe is present as glandular tissue applied to the
upper side of the ventral lobe. The mid-brain, which
>u« ceeds the thalamencephalon, bears above the two optic
lobes, which stand closer than those of the frog. The
cerebellum behind them is much larger than that of the
liog and oval in outline, with the long axis fore and
att, and overhangs the optic lobes in front and the thin-
roofed fourth ventricle in the medulla oblongata behind it.
1 he medulla is produced forward into a pair of wings,
the restiform bodies , which lie at the side of the
cerebellum.

The cranial nerves resemble in number and general
Nerves, distribution those ot the frog, but the presence

ot the gills and other differences in the arrange-
ment in the organs of the head causes the distribution to
differ in detail. The olfactory nerves are groups of fine
threads wrhich pass into the olfactory organs from the
adjoining olfactory lobes of the brain. The optic nerves
pass from the lower surface of the thalamencephalon,
each through the optic foramen of the opposite side,
to the eyeballs, crossing in a chiasma below the brain'
Flie third 01 oculomotor nerve of each side, arising from
tlic ventral surface of the mid-brain, passes outwards
through its foramen into the orbit of its own side, where
it supplies the superior, inferior, and internal recti muscles
ot the eye by short branches and gives a long branch across
the floor of the orbit to the inferior oblique. The slender
fourth, trochlear, or patheticus nerve arises from the dorsal
surface ot the brain between the optic lobes and the
cerebellum, and passes out through a special foramen to
supply the superior oblique muscle of its side. The sixth
or abducent nerve is also slender. It arises from the
ventral side of the medulla and supplies the external rectus
muscle, passing through the same foramen as the main
branches of the fifth and seventh nerves. The latter two
nerves, with the eighth, arise close together from the sides
of the medulla below the restiform body. The fifth or

THE DOGFISH

453

trigeminal has three branches. Of tnese the first, or
ophthalmic, parts at once from the rest ol the nerve, turns
forward within the skull, passes through a foramen in the
side of the cranium above the recti muscles, and runs
forwards along the outer side of the cranial wall, together
with the similar branch of the seventh nerve, to leave the

Fig. 338. — A diagram of certain cranial nerves in the dogfish. 1 he
nerves omitted (III, IV, VI) consist of motor hbres to the eye
muscles. Those shown supply the visceral arches and certain
other parts. They contain ( a , shaded) visceral motor (autonomic)
and visceral sensory fibres ; \b , black) fibres from the neuromast
system (p. 458) and ear ; and (c, white), chiefly in V, some other
somatic sensory fibres from the skin.

V.-X., Roots of the nerves; V.md., V.mx., V.op., mandibular, maxillary, and
superficial ophthalmic branches of the ffifth nerve ; Vop’., deep ophthalmic
nerve, not mentioned in the text, inconspicuous in the rough hound, but large
in many other fishes (p.465) ; VII. b., VII. e.m., VII. hd., Vll.i.m., VII.op.,
VI I. pal., buccal, external mandibular, hyoidean, internal mandibular, oph-
thalmic, and palatine branches of the seventh nerve; VIl.p.sp., VII.fn.sp.,
pre- and post-spiracular divisions of hyomandibular branch of seventh nerve ;
AM., X.v., lateral line and main visceral branches of the tenth nerve ; g.s., gill
slits ; m., mouth ; sp., spiracle

orbit by a foramen above the nasal capsule and be distri-
buted to the skin of the snout. The rest of the nerve leaves
the cranium by a large foramen below the recti muscles
and runs outwards across the orbital, floor as a broad band,
which divides into a maxillary branch to the upper jaw
and a mandibular branch to the lower. The seventh or
facial nerve has a complicated distribution : it possesses
(i) an ophthalmic branch which, leaving the cranium by a

454

MANUAL OF ELEMENTARY ZOOLOGY

foramen above the ophthalmic branch of the fifth, accom-
panies the latter ; (ii) a buccal branch, which joins the main
branch of the fifth within the skull, crosses the orbit with
it, leaves it before it divides, and is distributed to certain
sense organs (neuromast organs, p. 458) of the side of the
face ; (iii) a small palatine branch which runs across the
floor of the orbit behind the fifth nerve and supplies the
roof of the mouth ; and (iv) a large hyomandibular branch
which runs outwards in the hinder wall of the orbit and
passes down the hyoid arch. This branch gives off a small
prespiracular branch to the anterior wall of the spiracle,
after which it passes as- the postspiracular nerve behind the
spiracle and divides into three branches — an internal and
an external mandibular, and a hvoidean. Of these, it is the
internal mandibular which corresponds to the mandibular
(chorda tympani) of the frog. The palatine and hyo-
mandibular branches pass together through the same
foramen with the main part of the fifth. The eighth or
auditory nerve passes into the auditory capsule to supply
the inner ear. The ninth or glossopharyngeal nerve arises
from the side of the medulla behind and rather below the
eighth, passes through a passage in the cartilage of the
auditory capsule, emerges by its foramen behind the
capsule, and turns down the first branchial arch, after
giving off a small “ pretrematic ” branch to the hyoid arch.
The tenth or vagus nerve arises by a number of roots
immediately behind the ninth. It leaves the skull by a
foramen beside the occipital condyle, and runs backwards
along the anterior cardinal sinus, lying just median to that
vessel, immediately against its lining, through which it can
be seen if the vein be opened. It represents several nerves
fused, and gives off across the floor of the sinus a branch
to every branchial arch behind the first, each such branch
bearing a pretrematic branch to the preceding arch.
Shortly after leaving the skull the vagus gives off a lateral
line nerve, which runs along the side of the body, rather
deep among the muscles, and supplies an organ in the skin
known as the lateral line, which will be mentioned later.
After giving off the last of its branches to the branchial
arches, the vagus passes downwards to supply the heart
and other viscera (Fig. 321).

THE DOGFISH

455

These nerves and their principal branches may be
summarised as follows :

Name.

Function.

Distribution.

I. Olfactory

II. Optic ....

III. Oculomotor

IV. Trochlear

V. Trigeminal .

(a) Ophthalmic .

(£) Maxillary

(c) Mandibular .

VI. Abducent

VII Facial . ...

(a) Ophthalmic .

(d) Buccal .

(c) Palatine

(I) Hyomandibular .
VIII. Auditory

IX. Glossopharyngeal .

X. Vagus ....

(a) Lateral line .

(b) Branchial branches

(c) Main visceral .

Afferent

Afferent

Efferent

Efferent

M ixed

Afferent

Afferent

Mixed

Efferent

Mixed

Afferent

Afferent

Afferent

Mixed

Afferent

Mixed

Mixed

Afferent

Mixed

Mixed

Nasal organ.

Retina of eye.

1 oblique and 3 recti eye-muscles.
Superior oblique muscle.

Snout.

Upper jaw.

Mandibular arch (lower jaw).
External rectus muscle.

Snout (neuromast organs).

Side of head ( ,, ).

Roof of mouth.

Hyoid (and mandibular) arches.
Ear.

1st branchial (and hyoid) arches.

Lateral line sense organ.

Branchial arches 2-5.

Viscera.

The comparison of the cranial nerves with dorsal and
ventral roots of spinal nerves which was made with regard
to the frog (p. 92) holds good for the dogfish and all other
vertebrates. A feature of their distribution which was not
obvious in the latter animal is that certain of them (V., VII.,
IX. and X.) give branches, whose function is chiefly visceral,
to the arches. Each such branch gives off an afferent
pretrematic branch to the arch in front ot that, which it
chiefly serves (in the case of the fifth nerve this branch
passes to the upper jaw). The post-trematic branch is
efferent or mixed. The lay-out ot the primary components

of these nerves is shown in Fig. 338.

The spinal nerves of the dogfish are more numerous
than those of the frog, but in their general structure and
distribution resemble them. The dorsal and ventral roots
by which each arises from the spinal cord pass through
the wall of the neural canal by small notches in the hinder
edges of the intercalary pieces and neural arches re-
spectively. The sympathetic system is irregular and
difficult of dissection in the dogfish, but in the mam out-
lines of its plan it resembles that ot the irog.

456 MANUAL OF ELEMENTARY ZOOLOGY

Each of the olfactory organs of the dogfish (Figs. 332 and
sense Organs 3 3 6) is a sac enclosed in the olfactory capsule
of its side of the body. It opens externally by
the nostril, but has no internal opening into the moutht
Its walls are thrown into vertical folds covered with an
epithelium which contains sense cells. The eyes resemble in
all important respects those of the frog (p. 98), and need no.

FlG. 339.— The head of a dogfish, seen from above with the right 01 bit

opened.

e., Eyeball; o.i., o.s., inferior and superior oblique muscles', r.e., r.i., r.int., r.s.,
external, inferior, internal, and superior recti muscles', s/>., spiracle; //., optic
nerve; IV., fourth nerve.

here be described. On account, however, of their larger size,
they are more suitable objects for the study of the eye
muscles. Like the eyes of the frog and those of all other
vertebrate animals, each is moved by six muscles, which
arise from the inner wall of the orbit. Four of these, known
as recti, arise together near the hinder end of the orbit and
diverge to be inserted into the eyeball at various points.
The rectus superior runs outwards and forwards and is in-

THE DOGFISH

457

serted into the upper side of the eyeball. rl he rectus inferior
runs a similar course below the eyeball to be inserted into its
lower surface. The rectus inter nus or medialis runs forwards
between the eyeball and the cranial wall and is inserted
into the front side of the former. The rectus externus or
lateralis runs outwards behind the eyeball, into whose
hinder surface it is inserted. The remaining two muscles are

Pig. 340.— The left side of the head of a doghsh with the orbit opened

and the eye removed.

Orbitonasal foramen ; o.i., o.s., re., r.i.,r.im., r.s., eye muscles asinbig.339 »
s* a sniracular artery ; II -VII., cranial nerves ; III., tmrd nerve entering
the orbit and dividing to supply eye muscles ; III., its branch to the jnfenor
obliaue muscle • V.md., V.mx., V.op., mandibular, maxillary, and ophthalmic
branclies^/fifth n^rve; VII. km., VI I. op , VI I. pal., VII. fop ., hyomandibular,
ophthalmic, palatine, and prespiracular branches of seventh nerve.

on.

known as obliqui. They arise together near the anterior
end of the orbit and pass outwards and backwards to their
insertions into the eyeball. The obli([Uus superior is
inserted into the dorsal surface ol the eyeball just in trout
of the superior rectus ; the obliquus inferior is inserted in
a corresponding position in front of the insertion ot the
inferior rectus upon the lower side of the eyeball. By the
contraction of various combinations ol these muscles the

458 MANUAL OF ELEMENTARY ZOOLOGY

eyeball may be turned in any direction. The lower eyelid
is movable. The structure of the internal ear is essentially
similar to that of the frog (p. ioo). Its communication
with the external water and the absence of a drum have
already been mentioned (p. 426). Besides these sense
organs, which are found in all vertebrates, fishes possess a
peculiar system, known as the neuromast organs , which are
not found in any other adult vertebrates with the exception
of certain newls. These consist of sensory patches of the
epidermis containing sense cells, which bear short, stiff
sense hairs, and supporting cells. In the dogfish the
sense patches are placed at the bottom of tubes in the
skin, which are filled with slime or mucus. The most con-
spicuous of these tubes runs along the side of the body, its
position being marked by a rather indistinct lateral line
(f igs. 315, 317). It opens upon the surface of the body at
intervals. On reaching the head the lateral line divides
into two branches, which pass above and below the eye,
branch again, and rejoin in front upon the snout. Besides
this branching system of tubes there are, upon the snout,
others which pass straight inwards through the skin and end
m swellings or ampullae (Fig. 325, n.am.) which contain
sense patches. These can be found by pressing the skin
and thus squeezing the mucus out of them in little drops.
The neuromast organs are supplied by a special set of
nerve fibres, which join the same portion of the grey
matter of the brain with which the fibres of the auditory
nerve are connected, but enter the brain by various nerves
(Fig. 338), of which the principal are the ophthalmic
branch of the seventh and the lateral line branch of the
tenth nerve. The function of these organs is the detection
of vibrations in the water which are of too low a frequency
to be detected by the ear. The latter must be regarded as
a specially highly developed part of the same system as the
neuromast organs.

Finally, we may note the condition of the ductless
glands (p. 62) in the dogfish. With the
thyroid and the pituitary body we have
already dealt (pp. 444, 452). The thymus is
present as lobed masses of glandular tissue above the gill
clefts, from whose epithelium it arises during development,

Ductless

Glands.

THE DOGFISH

459

as in all vertebrates. The adrenal bodies are represented
by two separate elements. Between the. kidneys, an
elongate structure, the inter renal body , derived from the
c celomic epithelium, represents the cortex of the adrenals
of the frog and higher vertebrates ; on the course of t e
svmpathetic chains a number of bodies ot the same origin
as the cells of the sympathetic ganglia represent the
medulla of the adrenals. These bodies are the supra-
renaly, properly so-called, though that name is often
applied to the entire adrenal bodies

C H A P T e R XXII

COLD-BLOODED VERTEBRATA 1

The dogfish and the trog are examples of two of the
principal classes ot backboned animals — the
Cold- and ^ ishes, or Pisces , and the Amphibia . The

Warm-blooded, members of these groups, unlike the Birds
{d-ves)' and Suckling Animals, or Mammalia ,
which are the subjects of later chapters, are cold-blooded.'2

here exist two further classes of vertebrates, the Cvclosto-
mata and the Reptilia , both of which are also cold-blooded.
The cold-blooded vertebrates, other than the two which
we have already studied at some length, deserve our
attention both for their own sake and also because of the
light which is thrown by their anatomy upon that of the
examples we are studying in detail. We shall see that
^ome ot them link the trog to mammals, others bridge
the gap between the frog and the dogfish and lead down-
wards from the latter to the lancelet, and yet others
exhibit well developed, organs which are lacking or
insignificant m the dogfish and frog.

1 he Cyclostomes are a small group, ot low organisation,
Cyclostomata. whose members are fish-like but differ from all
other Vertebrata in that their mouth has no
jaws. Extinct members of this group ( Ostracoderms ) had
a strong bony armour in the skin and probably lived a
more active lite than their existing representatives the
Lampreys and Hags, which are semiparasitic creatures,

1 It has been assumed in writing this chapter that it will usually be

dL1( 10 [1 atter Chapter XXIV., but it is not necessary that this order
should be observed.

2 That is in them the temperature of the body varies with that of

aLScoldUbnioodSed. S“ PP' 74 and 557' Invertebrate animak

460

COLD-BLOODED I T ERTEBRA TA

46 r

without limbs, scales, or bony tissue, that live on juices-
obtained from their prey through a circular, suctorial
mouth. In place of the usual pair of nostrils, these

n. *.

Fig. 342. — A skate, dissected to show its skeleton and the position of
the alimentary canal in its abdominal cavity. — From Thomson.

c. Coracoid region of pectoral girdle; h.br., basibranchial cartilage; h.m hyo-
mandibular cartilage; M ., cartilage of lower jaw; m.pt., mesopterygiuin ;
mt.pt., metapterygium; n.c., nasal capsule ; propterygium; p.q., carti-
lage of upper jaw (palato-quadrate) ; pu., ischiopubic bar of pelvic girdle-
s.t., sensory tubules in the skin; s.v., spiral valve; sc., scapula; st., stomach :
v.Pl.t fused vertebrae. 462

COLD-BLOODED VERTEBRATA

463

creatures, and some of their extinct forbears, have a
single median opening which leads both to the olfactory
organ and also to a sac belonging to the pituitary body.
In the Lamprey (Petrowiyzon) this nostril is on the top of
the head. The Lamprey has an interesting larva, the
Ammoccetes , which feeds much as the lancelet does,
the thyroid gland opening on the floor of the pharynx
and, like the endostyle, secret-
ing mucus, which is carried
upwards by peripharyngeal
bands and backwards along
the roof of the pharynx to
the gullet. In the scientific
classification of the animal
kingdom the Cyclostomata are
separated from all other Ver-
tebrata, which, from their pos-
session of jaws, are known as
Gnathostomata.

Among fishes, three sub-
classes now exist
(see p. 471), the

DLll

p.c.s.

Fishes : Elas-

“hf Skate.' : Cartilaginous

Fishes or Elasmo-

Fjg. 343- — A semi-diagram-
matic view of the heart
and neighbouring vessels
of a skate.

a.b.a.t Afferent branchial arteries
a.c.s.y anterior cardinal sinus’,
au., auricle; c.a., conus arteri-
osus ; c.h.s., common hepatic

sinus; d.C., ductus Cuvieri ;
h.s'., hepatic sinuses ; p.c.s. ,
posterior cardinal sinus ; v.t
ventricle; v.clo., ventral aorta.

branchii , the Bony Fishes or
Actinopterygii , and the Lung
Fishes or Choanichthyes. The
Elasmobranchii have no bones
and no air-bladder, their gill
clefts are uncovered, and they
wear placoid scales (p. 422)-
Besides the dogfishes and .

sharks, skates and rays belong to this group. In
a skate, the body is transformed by an immense
development of the pectoral fins, which stretch forward
to the end of the snout and spread outwards so that
the width of the fish is much greater than its depth,
and by a narrowing of the tail to a whip-like organ
skate lives upon the ground, drawing its breaths of water
through the large spiracles, which are upon the top of the
head, whereas the gill clefts are on the smooth, white

464

MANUAL OF ELEMENTARY ZOOLOGY

under side with the mouth. The spiracle is used for taking
m water m the dogfish also, but is there not of the same

Cerebellum ; cer., cerebrum ; m.o., medulla oblongata • mam mandibular

6 lh/'L’ °.1.fac.t°ry lobe ’ °V-°;_ olfactory organ ; olf.st., stalk of olfactory
lobe , op.l., optic lobe ; os., superior oblique muscle ; r.b., restiform bodv •

rf> Tterna1' internal- and superior recti Muscles; S Tpiracle’
s Ac., spinal cord ; sp.n., spinal nerves (converging to form brachial plexus) •
II. X., cranial nerves ; V.m.m., common stem of maxillary and mandibular
branches of fifth nerve; l m.s. subsidiary branches in maxillary region-
Y. ;,superficial ophthalmic branch of fifth nerve, with which is united that of
seventh nerve ; V .op ., deep ophthalmic nerve ; Vll.b., inner buccal VII h'
outer buccal, VII. em., Vll.hd., Vll.i.m.., VH.p.sp. v^.pT vhpt^
external mandibular, hymdean, internal mandibular, prespiracular, palatine'
and postspiracular branches of seventh nerve ; X.g.s., XL, Xv gill slit lateral’
line, and main visceral branches of tenth nerve (vagus). ’ ^ 1 1 b lateral

importance, since the fish, not being flattened upon the
ground, can generally obtain most of its water through

COLD-BLOODED VERTEBRATA

465

the mouth. The arrangement of the principal blood-
vessels of the Common Skate, which differs a little from

that of the dogfish, is shown in
system (Fig. 344) there will be
seen to be two ophthalmic
nerves, one of which — the
superficial ophthalmic — passes
above certain of the eye
muscles (the internal and
superior recti and the superior
oblique), whereas the other —
the deep ophthalmic ( ophthal-
micus profundus')— passes be-
low them. The first of these
nerves corresponds to the two
ophthalmics which are con-
spicuous in the dogfish -the
ophthalmic branch of the
seventh cranial nerve and
the (superficial) ophthalmic
branch of the fifth — closely
united. The deep ophthalmic,
which is represented in the
dogfish by an inconspicuous
twig of the fifth nerve, sends
branches to the eyeball and
runs on to the snout.

Although it appears to arise
from the fifth, this nerve is really
the dorsal root of the nerve whose
ventral root is the third (p. 92). The
ophthalmic nerve of the frog is a
deep ophthalmic, though the super-
ficial ophthalmic, which does not
appear as such, is perhaps united
with it. The ophthalmic of a
mammal divides not far from its

Fig- 343- In the nervous

Fig. 345. — Transverse sections
through gill arches of a
dogfish (on the right) and
a cod. showing how Elasmo-
branchii differ from Actino-
pterygii and Choanichthyes
in respect of these organs. —
From Sedgwick, after R.
Hertwig.

a., Afferent, branchial artery ; b,
branchial arch of skeleton ; bl l
and bl-, gill lamellaB ; h, skin of
the side of the body between the
openings of two gill clefts in the
shark ; r, cartilaginous gill-ray
supporting the septum between
two gill pouches in the same ;
v, efferent branchial arteries,
double in the shark, single in the
cod ; z, small tooth-like tubercle
(in some Teleosteans elongated as
a “ gill raker ”), one of a double
row on the branchial arch of the
cod.

origin into a nasal branch, which

represents the deep nerve, and a frontal, which may be the superficial
nerve. The ophthalmic branch ot the seventh nerve (together with the
buccal and external mandibular branches of the same nerve and the

lateral line branch of the vagus, which also supply neuromast organs)
is lost, with the neuromast organs which it supplies, in the adults
of animals higher than fishes.

30

466 MANUAL OF ELEMENTARY ZOOLOGY

B

Branches of the lateral line system extend on to the
pectoral fins of the skate, and are connected with the
exterior by rather long tubules (Fig. 342). The anterior
vertebrae are fused into a continuous mass. Both ovaries
are present.

The Actinopterygii are not the only fishes which have
Actinopterygii bone as well as cartilage in the skeleton, but
they are the most bony. They have in the
abdominal cavity an air bladder which is a hydrostatic

organ, oxygen being secreted

/////// into or absorbed from it so

- as to alter the specific gravity

\\\\W\W of the fish ; a peculiar, thin,

non-nervous roof (pallium) to
the cerebrum ; and a bone-
supported fold, the operculum ,
over the external openings
of the gill clefts, which lead
straight outwards through the
sides of the throat, and do not
slantbackwardthroughpouches
like those of the Elasmo-
branchii (p. 435). Thus the
arches are narrow from within
outwards, and cannot accom-
modate the full width of the
gills, which project into the
chamber under the gill cover
(Fig. 345). The scales of the
Actinopterygii are flat, bony

A, Protocercal (Cycloseomata and Plate.S> “A )n a CaSeS

Dipnoi); B, Heterocercal (Elasmo* provided With Small Vestiges

Parac^caMT ; C' C" ' °f enamel-capped spines of the

placoid type, and are em-
bedded in the skin, usually overlapping like tiles on
a roof.

Certain small sections of the Actinopterygii, loosely
“Ganoids.” known as “ Ganoids,” of which the Sturgeon
(Acipenser) is an example, keep a varying
number of the following features of Elasmobranchii : the
spiracle, spiral valve, common vent or cloaca, muscular

eptcL

Fig. 347. — Actinopterygii.

A, The Sturgeon; B, the Crucian carp; C, the eel; D, scales on the skin of a
whiting, in surface view ; E, the same in section.

der., Dermis ; epid., epidermis ; m., mouth ; pc.f., pectoral fins ; pl.f., pelvic fins ;

sc., scales ; set., bony plates or scutes.

467

468 MANUAL OF ELEMENTARY ZOOLOGY

conus arteriosus (instead of a non-muscular “ bulbus
arteriosus 7'), and obviously unsymmetrical or “ hetero-
cercal 77 tail (the bony framework of the apparently sym-
metrical “ paracercal 77 tail of an ordinary fish is really
unsymmetrical, except in certain cases, such as the codr
etc., where, by the absence of the upturned tip of the back-
bone, it reaches a complete secondary symmetry). Most
ganoids have for scales stout, bony armour plates, over
which the epidermis wears away, very different from the
thin rounded cycloid scales which clothe the majority of
Actinopterygii.

Anus; «/C, af'L-i anal fins; b., barbule ; br.m., branchiostegal membrane
(a continuation of the gill cover); cf., caudal fin; dfP-df^., dorsal fins;
g. , genital opening ; na., nasal openings (double on each side) ; op., operculuis
or gill cover; p/., pectoral fin ; pvf., pelvic fin ; u., urinary opening.

All these features, however, disappear in the great mass.

of ordinary bony fishes or Teleostei. The
The waiting. Whiting ( Gadus meriangus), which, with its
near kindred the Haddock (G. ceglefinus ) and
Cod ( G . morrhua ), is often dissected in the laboratory,1 is
a typical Teleostean, both in the above respects and in the
very large number of bones which compose its complicated
skeleton (Figs. 544, 545). In it, as in many others, the
pelvic fins have shifted forwards till they lie actually in
front of the pectorals, and the air-bladder does not com-
municate with the gullet. The Salmon, Herring, Carp,

1 The student who wishes to study systematically one of these three
fishes will find on pp. 750, 751 a summary of its anatomy with addi-
tional figures.

COLD-BLOODED VERTEBRATA

409

and Eel resemble the ganoids in carrying their pelvic tins
in the normal position, and in that the air-bladder com-
municates with the gullet, which it does on the dorsal side.
Figs. 349 and 350 show the ventral and dorsal arterial
systems of the whiting. They should be compared with
those of the dogfish (Figs. 331 and 332), and the differences
noted, particularly the absence of an afferent vessel to the

Fig. 349. — A semi -dia-
grammatic ventral view
of the heart and neigh-
bouring blood-vessels
of a cod.

a.b.a., Afferent branchial
arteries ; a.c.s., anterior

cardinal sinus ; au., auricle ;
b.a., bulbus arteriosus ;
d.C., ductus Cuvieri ;
p.c.s., posterior cardinal
sinus ; v.. ventricle ; v.ao.,
ventral aorta.

Fig. 350 — A diagrammatic ventral
view of the dorsal arterial system
of a cod.

a.mes., anterior mesenteric artery ; a.psb.,
afferent pseudobranchials ; c.c., carotids ;
c.c'.} anastomosis between the internal caro-
tids which completes the circulus cephali-
cus ; coe., coeliac ; d.ao., dorsal aorta ;
e.b.a., efferent branchial ; e.c., orbitonasal
or “ external carotid ” ; e.psb., efferent
pseudobranchial ; i.c., internal carotid ;
op., ophthalmic ; p.c., orbital or “ posterior
carotid”; psb., pseudobranch; sbr., supra-
branchial ; scl., subclavian artery.

hyoid arch, which in the whiting carries no gill, but only a
vascular vestige or pseudobranch supplied from the efferent
system, and the replacement of the dorsal aorta in the gill
region by two suprabranchial arteries, which recall those
of Amphioxus. The whiting also reveals a feature in which
the Teleostei (and one or two ganoids) are unique among
Vertebrata— namely, that the oviducts are continuous
with the ovaries. The bony flat fishes (Soles, Plaice, etc.)
are related in general features to the whiting, but are

470

MANUAL OF ELEMENTARY ZOOLOGY

flattened — not, as is the skate, from above downward,
but from side to side in a remarkable way. They lie
on one side, which is white and has lost its eye, this being
brought on to the upper, coloured side by a twist of the
skull in the region of the orbits. The body is very tall —
that is, in its present position, broad. The young are
shaped like ordinary Ashes.

At this point we may pause to consider a relevant and
very interesting topic — the history of verte-
Verteb'ra^a^ °f brate animals. There is good reason to
believe that this began in inland waters. The
tact that the body of the lower vertebrata is so built as to

be a muscular, streamlined swimming machine suggests
that it came into being where there were strong constant
currents such as those ot rivers ■ and the glomeruli by
which the kidneys of vertebrata are enabled to excrete
water rapidly look like a defence against a peril which,
as we have seen, besets freshwater animals — the danger
of dilution ot the body fluids. Moreover, the earliest
remains of vertebrates are found in freshwater or
estuarine deposits. The first vertebrates were probably
cyclostomes, and a great advance was made when in

COLD-BLOODED VERTEBRATA

471

AA

some of them one of the gill arches came to strengthen the
borders of the mouth and thus made biting possible. (This
arch — the mandibular
arch — was not the fore-
most : there appear to have
been two clefts in front of
the present position of the
mouth.) Thus the true
Fishes, the earliest gna-
thostomes,came into being.

The first of them— the
Sub-class Aphetohyoidea ,
long since extinct— had
a full-sized cleft between
the mandibular and hyoid
arches, but before long
this cleft became reduced
so that it was possible for
the jaws (the mandibular
arch) to be held firm by
ligaments attaching them
to the hyomandibula.

Thus the modern fishes

arrived. Some of these passed into the sea and there,
after a time, for some reason bone disappeared from their

skeletons and they became
Elasmobranchs — thor-
oughly marine creatures
using the remarkable
expedient, which we
studied in the dogfish,
of retaining urea in
their blood so that they
do not lose water to
the medium around
them. The fishes that
remained in freshwaters
became more bony, lost
their gill pouches, and
developed opercula.
During the Devonian period freshwaters were subject to

Fig. 352A. — Aphetohyoid Fishes.

- — From Swinnerton.

(a) Climatius (Devonian) ; (b) Acanthodes
(Permian).

Both belong to the Order Acanthodi,
which had a strong spine in front of
each fin. Climatius, one of the oldest
known fishes, has on each side a row of
small fins from the pectoral to the pelvic,
and this suggests that in other fishes the
latter two fins are the remaining
members of a longitudinal series, which
perhaps arose by the break-up of an
original longitudinal fin on each side
like the long dorsal fin of some fishes.

— Crossopterygian Fishes.
From Swinnerton.

(a) Notoptychius ; ( b ) Glyptopanus. Note

the stout central portions of the fins,
containing skeleton and doubtless muscle,
and thus having the makings of legs.

472

MANUAL OF ELEMENTARY ZOOLOGY

periodic drying up, when much of that part of them which
continued to exist became foul and stagnant. In such con-
ditions some means of breathing air was essential. Those
fishes that survived this ordeal did so by gulping air from
the surface and using it for oxygenating their blood in the
lining of a pouch which they developed from their gullets
and which, as it grew, presently came to lie under the
backbone, where, as the air bladder, it now is in modern
bony fishes. Most of the fishes in which this happened
presently found another way out of the difficulty by going
down to the sea, where, when once they were acclimatised
to sea water, they found food plentiful and oxygen to be
had by their gills. Their air bladder, however, did not
degenerate but became the hydrostatic apparatus which
it now is. These fishes are the Actinopterygii. Most of
them have lost the duct that connected the air bladder
to the alimentary canal, but some keep it, and of them a
few can still use the bladder for respiration. These are
among those that, with the improvement of conditions in
freshwaters, have once more returned thither.

The fishes that remained to brave it out under the

choanichthyes unfavourable conditions of freshwaters in
Devonian times are known as the Choanich-
thyes. The better to breathe air by thrusting their snouts
out of water they adopted a new device. The oronasal
grooves which in the dogfish and its allies run from the
nostrils to the edge of the mouth are in Choanichthyes
closed over so that they have become tubes, and their
mouthward openings have shifted into the mouth, where
they become internal nares. Thus air can be drawn
through the nostrils into the mouth and so into the air
bladder, which is, in fact, a lung or, where it is divided,
a pair of lungs. The Choanichthyes fell into two groups —
the Crossopterygii and the Dipnoi. Of these one cross-
opterygian and three dipnoans alone survive. Both were
similarly adapted to breathing air, but the Dipnoi had a
peculiar configuration of the skull and teeth which enabled
them to crunch shellfish, and the fins of the Crossopterygii
were shorter and stouter. Nothing is known of the internal
anatomy of the Crossopterygii but they probably shared
wfith the Dipnoi certain features that point forward to the

COLD-BLOODED VERTEBRATA

473

Amphibia, notably special arrangements in the heart
and vessels to and from the lungs, paired cerebral hemi-

•' > • .> >' v) . )v< > A; A •

• 1 k > V ' S’ ' ' ) • ,9 l O') V. A * /i

' ; i" ; V • - Vi } %'MtM

k

B

Fig. 353. — Amphibians.

A, The Warty newt (Molge cristata) ; 1, female ; 2, male at the breeding season,
showing the crest which is specially developed at that time ; B, Coecilia, one of
the Gymnophiona. an., anus, in an enlarged view of the underside of the hinder
end. Note the absence of a tail.

spheres with roofs of nervous tissue, and a larva like the
tadpole.

Towards the end of the Devonian period Crossoptervgn
began to invade the land. Their powers of breathing and

474

MANUAL OF ELEMENTARY ZOOLOGY

their strong muscular fins, adaptable to a clumsy kind of
crawling, made this possible. The new adventure was
perhaps first undertaken when it became necessary to
wander from a drying pond to better waters, and extended
on account of the supply of food provided by the plant fife
which was then becoming plentiful on land and beginning
to support a population of arthropods. Land fife turned
the Crossopterygii that adopted it into Amphibia, the
first of the great group Tetrapoda — vertebrates that have
legs and five on land.

The Amphibians differ from fishes and agree with higher
Amphibia. vertebrates in two important respects — their
paired limbs are pentadactyle (p. 49), and if
they have unpaired fins these are without fin-rays. They

are also like the higher
vertebrates in possess-
ing lungs and an in-
ferior vena cava, but
the Dipnoi have what
are at least very pass-
able attempts at both
of these. Like the
fishes, however, Am-
phibia have only ten
cranial nerves, lay
eggs without shells,
lack the metanephros
and the embryonic
membranes known as
. the amnion and allan-

tois (p. 649), and start hie as gilled larvae. Most modern
Amphibia have no exoskeleton. The sturdy, long-legged,
tailless animals, known as Irogs and toads ( Anuro ), and the
long-bodied, short-legged, tailed Newts, or Urodelct. , are
naked but the. group also comprises the Gy mnop hiona — -
small blind, limbless, and tailless creatures, which five
like earthworms in the soil ol warm countries. The
Gymnophiona have rings of small scales embedded in the
skin, recalling those ot the Teleostei, but probably really
the last remains ol a scaly armour that covered parts of
the body ot certain members of the earliest groups of

Fig.

354- — Stegocephalia. — From
Swinnerton.

(a) Mastodonsaurus (Upper Trias) ; ( b ) Cacops
(Permian).

COLD-BLOODED VERTEBRATA

475

Amphibia ( Stegocephali ), now extinct, Upon the head
of the Stegocephali as on that of the Crossopterygu the
scales were replaced by bony plates that became a part ot
the skull, which was in other respects more highh
developed than that of the frog. The skulls ot modern
Amphibia show a progressive simplification in Gyinno-

phiona, frogs, and newts. 1 . ,

The reader will find on pp. 632-633 of this book an
account of the fish-like arrangement of the arteries oi. a
tadpole, and their relation to that of a frog. Here it may
be added that in various newts more than one pair ot
aortic arches persists in the adult. In the Common Newt
(. Molge ) the ductus arteriosus remains well developed,
giving an additional pair ot arches (the pulmonary) , . m
the Salamander the missing third branchial arch, lying
between the two that are persistent in the Common Newt
is also complete ; in other cases both ductus caroticus and

ductus arteriosus remain open, and so on.

In the newts there is a distinction between two regions
of the adult kidneys such as is found m the dogfish but

not in frogs and toads (see Fig. 353). .

The Amphibia are but amateurs at terrestrial hie.

Their not very efficient apparatus lor breathing
Repti,ia- by forcing air into their lungs is supplemented

by using the" skin, which has therefore to be permeable,
and consequently they lose much water through it when
they are on land. What is even more important is that
their eggs are laid as spawn, which must be kept wet, and
that their gilled larvae must live in the water. It is not
surprising that such creatures are less numerous and
successful than either the fishes, perfectly adapted to life
in water, or the Reptilia, the next group oi vertebrates to
appear, which are thoroughly land-adapted animals.
Reptiles1 resemble Amphibians in being cold-blooded,
lung-breathing, pentadactyle vertebrates, but they difiei
in a number of ways, in most ol which they are the better
fitted for a life on land. They have an impermeable exo-
skeleton of horny scales, with sometimes also bony plates :

1 The student who wishes to make a systematic study of the anatomy
of a reptile will find assistance, with figures, m the practical direc-
tions, on p. 754, for the examination oi a lizard.

476 MANUAL OF ELEMENTARY ZOOLOGY

their lungs are filled and emptied by a process of ex-
pansion and contraction which is more efficient than the
injection procedure of the frog ; the heart and great
arteries are better adapted to carry on the double circula-
tion through the lungs and body ; their eggs are pro-
tected by a chalky shell, in which the embryo is saved
from drying up by being enclosed in a sac of watery fluid,
the amnion , and is provided with an organ (the allantois )
by which it breathes through the porous shell ; they have
no larval stage but by a great store of nourishment (yolk)
in the egg are enabled to remain in the shell until they
have taken on the adult form. They have twelve pairs of
cranial nerves,1 and their functional kidney is the meta-

nephros (pp. 548, 636). Exist-
ing reptiles crawl with their
bellies on the ground, like
amphibians and unlike birds
and mammals, which walk
with bodies raised by their
legs, since these are not only
bent downwards at the middle
joint (Fig. 359 A) but also have
rotated inwards so as to stand
under the flanks, parallel with
the body. Certain extinct reptiles resembled birds and
mammals in this respect.

The paired limbs of vertebrates act as levers ; those of the penta-
Walking dactyle type are compound levers composed of several

segments jointed together, and are thus the better able
to execute complicated movements and can lengthen by straightening.
In walking they work essentially as follows : certain of them, being
straightened, raise the body higher above the ground and tilt it
onwards ; meanwhile the others, kept bent, moved partly by muscles,
partly by the mere effect of gravity, swing forwards and take up a
position in front ot those that have remained upon the ground ;
then those that have swung touch the ground, straighten, and in
turn allow the rest to swing forwards. In four-legged animals it is
usually the near fore leg and the off hind leg which work together
(p. 56) ; in those which walk upon two legs the pair act alternately
(Fig- 453)-

The Reptiles supplanted the Amphibia as the dominant
animals of the land, and many and various kinds of them

1 Except in snakes.

COLD-BLOODED VERTEBRATA

.4

4/ /

came into existence, including some that flew and some
that were as well adapted to marine life as the whales.
But at the end ot the

mesozoic period they de-
clined before the advance
of tw'O groups that arose
f rom amon g them — t he
Birds and the Mammalia.
These groups are quite un-
related ; indeed each of
them resembles certain ex-
tinct reptiles more than it
does the other ; but they
have both solved a problem
before which the reptiles
failed. They are “ warm-
blooded ” ; that is, by keep-
ing the temperature in their
bodies constant they ensure

that their activity is neither
slowed by cold nor dis-
ordered by excessive
warmth. Thus in them
one more of the major
factors of the environment
has been mastered. The
existing members of the
Reptilia are : d urtles,

Crocodiles, Snakes, Lizards,
and the Tuatara, a little
lizard-like animal found in
New' Zealand.

The skulls of reptiles are com-
plicated structures. They contain
less cartilage than those we have
dealt with as yet, and the arrange-
ment of the bones is of interest
in several ways. (i) In the .

occipital region, there are, as well as the exoccipitals at the sides,
a supra occipital above and a basioccipital below, and (except
in some extinct forms) a single occipital condyle underneath the
foramen magnum replaces the two lateral ones of the frog. We

Fig. 356.— The skull of Capita
saurus nasutus , one of the
Stegocephali. — f rom Rey-
nolds, after von Zittel.

1, Premaxilla; 2, nasal; 3, maxilla;
anterior nares ; 5, frontal ; 6, pre-
frontal ( or lachrymal) ; 7, lachrymal
{or adlai hrymal) ; 8, jugal ; 9, orbit ,
ro, parietal ; ri, postfrontal ; 12, post-
orbital ; 13, interparietal foramen ;

14, supratemporal ; 15, squamosal ;

16, quadratojugal ; 17, quadrate;

18, tabulate ; 19, postparietal ; 20,

exoccipital ; 21, foramen magnum.

The sheet of membrane bones which
forms the roof of this skull is a special
development of the armour of bony scales
which is found on other parts of the body
of Stegocephali. It is not only the roof of
the cranium, but stretches over the space
between the cranium and the upper jaw
(palato-pterygo-quadrate bar). In most
other bony skulls, gaps (the fossae, p. 44°>
appear between the bones of this dermal
sheet.

478 MANUAL OF ELEMENTARY ZOOLOGY

shall see later that birds have one condyle, and mammals two.
(2) 1 he trontoparietals of the frog are usually represented here,
as in most other vertebrates, by separate frontal and parietal
bones. Prefrontals and postfrontals lie at the corners of the
frontals. (3) In most cases (not in snakes) the forepart of the cranium
is so compressed between the eyes that its hollow disappears, and it
is replaced by a vertical sheet of membrane, the interorbital septum,
so that in the dried skull the two orbits open widely into one another.
The sphenethmoid of the frog is not found. (4) The quadrate
cartilage of the frog is replaced by a quadrate bone, with which the
lower jaw articulates. This bone is tound in birds, but not in mammals,
where the lower jaw articulates with the squamosal. (5) Except in
turtles a transpalatine bone joins the maxilla to the pterygoid. (6) The
true cranium is not large, but there is a scaffolding about it which

Tig. 357-— Semi-diagrammatic views of the arterial arches.

/I, of a salamander ; B, of a common newt.

l~4, V™?** the,first l? fourth branchial arches ; c.gl., carotid gland ; car., caro-
I L fnb™!]’ coeliac artery ; d.ao., dorsal aorta ; d.ar., ductus arteriosus ;
AN ,1?ternal car°Gd artery; Ing., hngual artery; p., pulmonary artery;
systetStc ar0chary 5 " truncus arteriosus ; scl., subclavian artery ; sy.,

supports the jaws, and upon which the skin of the head is stretched.
This is the remains of a complete false roof ot dermal bones found in
the Stegocephali (p. 475), which is now under the skin. The more
median of these bones form the roof of the cranium ; those at the
sides, arching outwards to the upper jaw, enclose a cavity which is
filled by the jaw muscles. The earliest reptiles had this roof
complete and it is so in turtles, but in the remaining reptiles (and
in all other land vertebrates) it is pierced by openings, known
as fossa, which give more room for the muscles. The scaffolding
is most perfectly developed in the tuatara. Here there are two
longitudinal bars or arcades strutted out from the skull by two
transverse bars. The bars are composed as follows : (i) The upper
or supr atempor al arcade, , parallel with the parietal region of the
cranium, ot the postorbital and squamosal (in other reptiles the
postorbital becomes a part of the postfrontal ) ; (ii) the lower or
infratemporal arcade, below and outside the supratemporal, of the

COLD-BLOODED VERTEBRATA

479

jugal and quadratojugal ; (iii) the postorbital bar , of the postfrontal,
postorbital, and jugal ; (iv) the post -temp oral bar at the hinder end
of the skull of the parietal and squamosal (with which is fused a

A, Female ; B, male.

cl cloaca* f.b., fav body; i.o.d., internal opening of oviduct ;
W" ? anterior part of kidney; k\, posterior part; k.d., primitive kidney duct
(Wolffian duct) ; od., oviduct ; od'., vestige of same in male ; ov., ovary ; rm.,
rectum ; U, testis ; ur., ureter ; v.eff., vasa efierentia, or vestige of same m

female.

480

MANUAL OF ELEMENTAL V ZOOLOGY

B

Fig. 359. — Two English reptiles.— From Lindsay.

A, The Sand Lizard, Laccrta a^ilis B, the Viper, Pcltas berut.

COLD-BLOODED VERTEBRATA

481

supratemporal, separate in lizards). The fossae are : the supra-
temporal fossa , between the cranium and the supratemporal arcade
bounded by the postorbital bar in front and the post-temporal behind
and the infratemporal fossa, similarly placed between the two arcades.

A similar arrangement is found in crocodiles, where, however, the
bones are stouter and the fossae smaller. In lizards there is a single
fossa. Probably this is the supratemporal, the infratemporal arcade
and fossa having disappeared owing to loss of the quadratojugal
(Figs. 361 B and 549). In snakes (and some lizards) the supratemporal
arcade and fossa also disappear. The result of these dispositions is
that in crocodiles, turtles, and the tuatara the upper jaw is held rigid,
in lizards it has a good deal more elasticity, and in snakes it is ex-
tremely elastic, so that the animal can swallow very large prey. The
elasticity of the jaws of snakes is further increased by the absence in
them of a process from the exoccipital which in other reptiles struts

Fig. 360. — A section through the skin of a lizard. — From Shipley and

MacBride.

1, “ Epitrichial layer” of clear cells; 2, heavily cornified cells forming the scale;
3, pigment cell ; 4, ordinary cells of horny layer ; 5, innermost Malpighian
layer ; 6, dermis.

the quadrate. If with these arrangements in reptiles we compare
that in a frog, we shall find that the lower arcade is present, but that
the absence of the upper arcade and the postorbital bar leaves one
large fossa at the side of the cranium. The same is the case in birds.
Mammals have an arcade, the zygomatic arch, composed mainly
of an element, the jugal, which belongs to the lower arcade of the
reptiles. Above this they have a single temporal fossa.

The heart of reptiles consists of sinus venosus, right and left
auricles, and ventricle partly divided by an incomplete septum into
right and left chambers, except in crocodiles, where the division is
complete, so that there are two ventricles as in birds and mammals.
There is no conus arteriosus, and the pulmonary artery and right and
left systemic arches communicate with the ventricle each by an
opening of its own, though the svstemics may adhere outwardly to
form a so-called truncus arteriosus. They cross at their bases, and

31

482

MANUAL OF ELEMENTARY ZOOLOGY

in crocodiles the right arch arises from the left ventricle and the left
arch, with the pulmonary, from the right ventricle. The right is the

Fig. 361. — Diagrams of the arcades and fossae of the skull.

A. In the tuatara and crocodiles (“ diapsid ” type) ; B, in most lizards ;

C, in snakes ; D , in the frog ; E, in mammals (“ synapsid ” type).

The bones of the cranium are shaded heavily, those of the arcades lightly.

e.n., External nares ; e.n'., the same in crocodiles ; the single fossa in

lizards, frogs, and mammals, respectively ; f.p., fronto parietal ; fr., frontal ;
i.t.f., infratemporal fossa ; ju., jugal ; mx., maxilla ; n., nasal ; orb.,
orbit ; par., parietal ; p.m, premaxilla ; prf., prefrontal ; pto., postorbital
with postfrontal ; q.j., quadratojugal ; s.t.f., supratemporal fossa ; sq.,
squamosal.

Compare Fig. 362.

more important channel of the two, and conveys the arterial blood.
These dispositions foreshadow that of the birds, in which the right
systemic arch alone persists. (The only complete aortic arch of a
mammal is the left systemic.) In most lizards (Fig. 54S) the ductus

Fig. 362.— Skulls of reptiles.

A , Dorsal view of the skull of the tuatara ( Sphenodon ) ; B, the same view of the
’ skull of the grass snake ( Tropidonotus natrix ), with small portions of the lower
jaw ; C , dorsal, and C , ventral views of the skull of a crocodile.
a.n., Anterior nares ; a.pv ., anterior palatine vacuity; b.oc., basioccipital ; Ed.,
common opening of Eustachian tubes ; epipt., epipterygoid ; ex.oc ., exoccipitai ;
J.m., foramen magnum ; fr, ., frontal ; ju., jugal; lj., portion of lower jaw;
l.t.f., lateral temporal fossa; lac., lachrymal; nix., maxilla; nasal; n.i.,
notch into which fits the fourth tooth of the lower jaw; o.i., opening into which
fits the first tooth of the lower jaw ; o.c ., occipital condyle ; orb., orbit \p.n., pos-
terior nares; p.p.v., posterior palatine vacuity ; par., parietal;//., palatine;
pm., premaxilla; prf., prefrontal; pro., prootic; pt., pterygoid \ ptf., post-
frontal; pio., postorbital ; q., quadrate; q.j., quadratojugal ; s.oc., supra-
occipital; sq., squamosal; st.f., supratemporal foramen; tpt., transpalatine.

For the lizard and turtle, see Figs. 549 and 366. ,

Note that the tuatara and crocodiles have both supratemporal and lateral temporal
fossm, the lizard, being without quadratojugal, has no lateral temporal fossa, the
snake' has neither arcade, and in the turtle there are no fossae.

Compare Figs. 356, 361.

483

484 MANUAL OF ELEMENTARY ZOOLOGY

caroticus persists, and in turtles the ductus arteriosus. The venous
system of a reptile is much like that of an amphibian, and has a
renal portal system, which is lacking in birds and mammals.1 The
red blood corpuscles are oval and nucleated, like those of other cold-
blooded vertebrates and birds, not like those of mammals. Turtles,
whose ribs are fixed by their shell (p. 486), breathe by movements
of the pectoral girdle, other reptiles breathe by movements ot the
ribs like those of mammals (p. 547)1 T>ut there is no midriff. (The
active movement in the breathing ot a bird is that by which the air
is driven out.) There is no important difference between the reptilian
and amphibian nervous systems, but by the taking into the brain of
the hypoglossal, and the appearance of a nerve between it and the
vagus, the number of cranial nerves is raised to twelve. The functional
kidney of reptiles is the metanephros (p. 636). The mesonephros,
though it has quite lost its urinary function, persists in the male as a
body called the epididymis attached to the testis, and from it the vas
deterens leads. Thus here, as in all vertebrates except Cyclostomes
and Teleostei, the testis discharges its products through a part of the
original kidney, and uses as its vas deferens the original kidney duct
(Wolffian duct : see Fig. 415).

Orders of
Reptiles.

We will now briefly survey the surviving groups ot
reptiles. Lizards (. Lace rt ilia ) and Snakes
( Ophidia ) are closely related : indeed, it is
difficult to distinguish between them, foT there
are lizards, like the Blindworm, which have no outward
trace of limbs, and snakes, like the Python, which have
vestiges of hind legs. The true snakes, however, may be
distinguished by the absence of any trace of a shoulder
girdle or urinary bladder. The shoulder girdle of lizards
(Fig. 550) shows a bony scapula, coracoid, and precoracoid,
with a cartilaginous epicoracoid, meeting a broad cartila-
ginous sternum, to which some of the ribs are usually
prolonged, as in birds and mammals, but not in amphi-
bians. There are also clavicles and a long, median
inter clavicle, which lies upon the sternum. Both snakes
and lizards have two penes — hollow sacs which open into
the hinder wall of the cloaca, and can be protruded by
being turned inside out.

A snake crawls or climbs by means ot its ribs. 1 hese are
attached to broad scales on the belly (seen in Fig. 359 B).
which they raise in turn, and thus cause to grip the surface
over which the animal 'is moving. The backbone is enabled

1 A comparison of the principal types of venous system found in
Vertebrata is given in Fig. 421.

COLD-BLOODED VERTEBRA, TA 4§5

to stand the strain of this process by the presence on each
vertebra of two knobs, the zygosphenes , which fit into pits,
the zygantra, on the hinder side of the preceding vertebra.

Turtles and tortoises ( Chelonia ) are characterised by
having bony shields ( carapace and plastron) on back and
belly, the absence of a sternum, the peculiarities of their
strong, toothless skulls, and the presence of a single

486 MANUAL OF ELEMENTARY ZOOLOGY

B

-4

penis. In the turtles the hands and feet are converted
into paddles, in tortoises they are adapted for crawl-
ing. In some turtles the Skeleton of the limbs shows all
the bones of the typical pfintadactyle limb (Fig. 24). The
only other case in which this is found is that of the hind
limb of certain newts. The sharp, bony jaws are covered
with horny plates and are very effective both for feeding and
as weapons, forming, as they do, part of a very rigid skull.

The carapace is composed
of a number of dermal
bones, some of which are
fused to parts of the endo-
skeleton. In the middle
is a row of small neural
plates , fused to the neural
spines of vertebrae. On
each side of this row is a
set of costal plates , fused
to the ribs. A ring of
marginal plates , com-
pleted in front by a
nuchal plate , outlines the
whole, and some little
pygal plates fill a gap at
the hinder end of the
neural series. Since the
plates of the carapace are
sutured together, the ribs
and vertebrae which are
fused to them are im-
movable. The plastron
is also composed of
dermal bones, some of which represent the clavicles
and interclavicle of other reptiles. Over the cara-

pace and plastron the scales are represented by large,
horny, epidermal scales or shields of “ tortoise-shell.”

arranged in a regular pattern but not corresponding to the
underlying bones. Nerve endings give the tortoise-shell
a certain sensitiveness. The two halves of the shell are
strutted by a characteristic tripod-shaped shoulder girdle,
in which the precoracoid has disappeared, its place being

Fig. 364* — A dorsal view of the
carapace of a turtle. — From
Reynolds, after Owen.

1, Nuchal plate; 2, first neural plate; 3,
second costal plate ; 4, marginal plate ;
5, pygal plates ; 6, rib ; 8 and 9, outlines
of first vertebral and third costal epi-
dermic shields.

COLD-BLOODED VERTEBRATA

487

taken by a well-developed acromion process. We shall
find this structure, small in the frog, to be well developed
in mammals. No doubt the plastron makes a sternum
needless.

Crocodiles, alligators, and gavials ( Crocodilia ) are lizard-
shaped reptiles with bony dermal plates corresponding to
the epidermal scales of the back, two arcades and a fixed
quadrate in the skull, a palate separating nasal passages
from the mouth, a sternum and pectoral girdle, one penis,
no bladder, and completely separated ventricles. The

488

MANUAL OF ELEMENTARY ZOOLOGY

palate is a structure we have not yet met with. It is
supported by flanges of the premaxillse, maxillae,
palatines, and pterygoids arching under the primary
roof of the mouth and forming a secondary roof. A
short partition of this kind, not involving the pterygoids,
is found in turtles. In mammals there is a palate longer
than that of the turtles, but not so long as that of crocodiles
(see p. 539).

Fig. 366. — The skull of a turtle. — From Thomson.

AN., Angular; AR., articular; D., dentary; FR., frontal; J., jugal;
MX., maxilla ; PF., prefrontal ; PAR., parietal ; PMX., premax dJa ;
POF., postorbital fused with postfrontal ; Q., quadrate ; QJ., quadrato-
jugal ; S., surangular ; SO., supraoccipital ; SQ., squamosal/

The Tuatara ( Sphenodon ) is the only living representative
ot the Rhynchocephalia , a very ancient group of reptiles,
lizard-shaped animals without bony armour, with two
arcades and a fixed quadrate in the skull, no penis, and
an incompletely divided ventricle. The pineal organ of
Sphenodon is more highly developed than in any other
existing vertebrate, and its distal part shows a striking
resemblance in structure to an eye and lies in a foramen
on the roof of the skull between the parietal bones.

COLD-BLOODED VERTEBRAE A

4S9

The same arrangement is seen in lizards, but less well

developed. ,

We have seen that two warm-blooded groups ot animals
took over from the reptiles the dominance ot

Birds and the world. Both of them are warmly clad, one

Mammals. wkh feathers? the other with hair. Both owe

their success to their activity and resourcefulness, but
these characters are shown very differently by them. I he
Birds have an organisation which in almost every detail
adapts them, directly or indirectly, to flight. 1 heir
behaviour shows a very high development ot instinct—

pIG_ 357 .—The Tuatara (, Sphenodon punctatus ), a lizard-like reptile
found onlv on some islets off the coast of New Zealand.

fixed conduct for particular emergencies. They have suc-
ceeded by specialisation. The organisation and behaviour
of mammals are more plastic. Members ot this group,
though some of them can fly, are less efficient in the air than
the highly adapted birds, but elsewhere they are dominant.
This they have achieved partly by suitable modifications ol
their basic organisation but even more by their adaptable
behaviour. Both they and the birds tend their young, but
mammals do this more efficiently in that all ot them provide
a special food— milk— and nearly all give the ova protection
and nourishment within the body ot the mother during
development. We shall now study a typical example ot
each of these groups.

CHAPTER XXIII

THE PIGEON 1

The many different kinds of domestic pigeons are familiar
to every one. All of them — carriers, tumblers,
The Rock Dove. fantajjs> pouters, etc. — have been bred, by

selection continued for many generations, from the wild
Rock Dove ( Columba livid ), a bird of strong flight which is
found over a great part of Europe and Asia, building among

Fig. 368. — A pigeon wheeling in the air. — From Pettigrew.

The bird is steering to its left. Note the different positions of the wings and the

spreading of the tail feathers.

high rocks or in ruins an untidy nest of sticks, in which two
white eggs are laid. It feeds on seeds of various kinds.

The pigeon’s boat-shaped body offers little resistance
to the air, and to the same end has an even ,
contour, due to the coat of feathers, which also
affords a light and warm covering. A distinct
head, neck, and trunk are present, but the tail is a mere

1 It has been assumed, in writing this chapter, that the pigeon will
usually be studied after the rabbit.

490

External

Features

THE PIGEON

491

stump which bears a fan of long feathers. Since the fore-
limbs are wings, the hinder — the legs — must support the
whole weight in standing. We shall see that the skeleton is
adapted to this necessity. The feet are naked and covered
with scales, which are horny and epidermic like those of

Fig. 369 — A plucked pigeon, seen in dorsal view.

upt.., Apteria ; cr., cere ; ear ; na., nostril ; o.g.p., papilla on which the oil gland

opens ; ptl., pterylae ; px., thumb.

a reptile, not like those of a fish. There are four toes,
which have a wide tread, the first being directed backwards
and the other three forwards ; the fifth is wanting. The
front or facial portion of the head is drawn out into a beak
covered with horny skin. At its base above is a swollen,
featherless patch of skin, the cere. The nostrils lie below
the cere, the eyes behind it at the sides of the head, and the

492

MANUAL OF ELEMENTARY ZOOLOGY

ear openings below and behind the eyes, covered by
feathers. There are three movable eyelids (p.33), and the
drum of the ear is at the bottom of a tube, but there is no
ear flap. There is a single cloacal opening, as a trans-
verse slit below the tail, and above the tail is a knob on

Fig. 370 — A plucked pigeon, seen in ventral view.

apt., Apteria; cl., cloacal opening; cr., cere; ear ; na., nostril; ptl., pteryla ;

px., thumb ; scl., scales on the foot.

which opens the oil gland, whose secretion is used in
preening the feathers.

The feathers are epidermal structures. When the bird
is plucked they are found to have been arranged
in certain tracts or pterylce, leaving between
them bare apteria, The feathers are of several kinds. The

THE PIGEON

493

quili feathers are found along the hinder edges of the
wings and tail, those on the wings being remiges , those on
the tail rectrices. The contour feathers cover the body.
Those at the bases of the quill feathers are known
as coverts . Filoplutnes are little hair-like feathers
among the contour feathers. The feathers are moulted

Fig. 371. — A diagram of a developing feather, highly magnified.

— From Shipley and MacBride.

der., Dermis ; epid., epidermis ; fol., follicle ; fth., feather ; Mp., Malpighian
layer of epidermis ; pap., papilla by the growth of whose epidermis the feather
is formed.

every' year and thus those damaged by use become
replaced.

A quill feather consists of the following parts : The stem or scapus
is divided into a lower, hollow part, the calamus or quill, and an upper,
solid part, the rachis . The quill is embedded in a pit of the skin and
has at its lower end an opening, the inferior umbilicus , through which
a vascular papilla projects into the growing feather. At the junction

494

MANUAL OF ELEMENTAR\ ZOOLOGY

In the wing of a plucked
bird there may
?i!ght.and easily be made
out parts corre-
sponding to the upper arm,
forearm, and hand. In the
latter the thumb is the only
digit that projects. A fold
of skin known as the pro-
patagium connects the
shoulder with the forearm in

Fig. 372. — Feathers of a pigeon.

A, Down feather; B, filoplume ; C,
quill feather.

d.s., Aftershaft; i.u., inferior umbilicus ;
qu., quill or calamus ; rch., rachis or
shaft; s.u., superior umbilicus; vex.,
vexillum or vane.

i.tt.

of the quill and rachis is a minute opening known as the superior
umbilicus. Close to this arises a small tuft known as the aftershaft .
The rachis is the axis of the flattened part of the feather known
as the vexillum or vane. This is
composed of a series of elastic
plates set along the sides of the
rachis with their flat sides perpen-
dicular to the plane of the vane.

The plates are known as barbs ,
and they are held together by bar-
bules , which are smaller processes
that fringe the barbs. The bar-
bules of one side of a barb (distal
barbules) bear little hooks or bar-
bicels which catch upon the bar-
bules of the adjoining barb. Thus
the whole vane is held together and
forms a single surface for striking
the air. The barbules of the
contour feathers are less well de-
veloped than those of the quill
feathers, so that the barbs separate
more easily. The filoplumes con-
sist each of a hair-like stem with
a very rudimentary vane of a few
isolated barbs at its apex.

C

B

A

THE PIGEON

495

front, and a small postpatagium of the same kind lies across
the armpit. The greater part of the surface of the wing,
however, is provided by the row of twenty-three remiges
along the hinderside of the limb. The remiges borne
upon the hand are eleven in number and are known as
primaries . Those on the forearm are known as secondaries .
A tuft of feathers on the thumb is the bastard wing. In
flight, as the wing strikes downwards its strong front edge

Fig. 373. — Parts of a feather. — After Nitzsch.

I., Four barbs ( B .) bearing anterior barbules ( A.BB .) and posterior
barbules ( P.BB .) ; II., six barbs {B.) in section, showing inter-
locking of barbules ; III., anterior barbule with barbicels (H.).

is twisted forwards so that the concave lower surface faces
back as well as down and thus the body is both propelled
and supported, much as we saw that of the cockroach to
be (pp. 327, 328). In rising flight the angle is altered so
that the wing presses more downwards. In gliding, the
wings are outspread and serve as those of an aeroplane.
The tail feathers can be spread out on one or both sides,
and are used for steering, and to check the “ way ” ol the
bird, as in alighting. The downstroke ol the wing is more
powerful than the upstroke, which is helped by the fall of
the body when the downstroke ceases to raise it.

MANUAL OF ELEMENTARY ZOOLOGY

496

Fig. 374. — Two positions in the flight of a pigeon. —

From Marey.

Below : shortly after the beginning of the downstroke. Above : near the end of the

same stroke.

THE PIGEON

497

The pigeon is a backboned animal, and its structure is
on the same general plan as that of the frog and dogfish.

It has a chest or thorax, walled by ribs and a broad breast-
bone, but lacks the midriff or diaphragm of
murntai mammals. It is of the pentadactyle type and
skfitton. its skeleton resembles in main outlines that
of the frog. The bones are very light and
spongy in texture, and most of them, except those of the
32

498 MANUAL OF ELEMENTARY ZOOLOGY

tail, forearm, hand, and hind-limb, contain air spaces. A
tendency to the fusion of bones is seen in various regions.

Fig. 376. — The skeleton of a pigeon, seen from the left side.

C r , Fixed cervical rib; c.r'., free cervical ribs ; cl., clavicle ; cor., coracoid ; d.,
dentary; Eu., Eustachian tube; e.oc., exoccipital ; f.r., fenestral recess ; Je.,
femur ; fi., fibula ; fr., frontal ; hu., humerus ; i.J., iliosciatic foramen ; i.o.s.,
interorbital septum ; il., ilium ; is., ischium ; lac., tacrymal ; me. 1-3, metacar-
pals ; ml. 1-4, metatarsals; n., nasal; o.f., obturator foramen ; pa., patella;
par., parietal ; ph., 1-4, phalanges ; pi., palatine ; pm., premaxilla ; p.o.p.,
postorbital process of frontal ; pt., pterygoid ; pu., pubis ; pyg., pygostyle ; q.,
quadrate ; r.c ., radial carpal ; ra., radius ; s.o.b., suborbital bar ; s.oc., supra-
occipital ; sa., supra-angular ; sc., scapula ; sq>, squamosal ; si., Sternum ; sl.r.,
sternal ribs ; ti., tibia ; u.c., ulnar carpal; u.p., uncinate process ; ul., ulna ;
v.cd., caudal vertebrae ; P.r., vertebral rib ; x xiphoid process.; syt, zygomatic
process of the squamosal ; /., II., foramina for first two cranial nerves ; 1-3,
first three cervical -vertebrae.

THE PIGEON

499

and the proportion of cartilage is very small. The back-
bone is divided into five regions : (i) The neck contains
thirteen to fifteen cervical vertebrae, the commonest number
being fourteen. The ends of the centra of these are of a
peculiar shape known as heteroccdous. In front they are
saddle-shaped y concave from side to side, and convex from
above downwards : behind they have these curvatures
reversed. The third to the eleventh or twelfth cervical
vertebrae bear short ribs fused to the centra and trans-
verse processes. The ribs of the last two are free, but do
not reach the breastbone. (2) Behind these come five
thoracic vertebrae, whose ribs reach the breastbone. Of
these the first three are fused together, the fourth is free,

Fig. 377 — Cervical vertebrae of a pigeon.

A, From in front ; B, from behind.

az., Prezygapophysis ; c.r., cervical rib; cm., centrum; n.a., neural arch; p.z.,
postzygapophysis ; ver.c., foramen of transverse process.

and the fifth is fused with those behind it. (3) The next
half-dozen vertebrae are known as lumbars and are fused
in front with the last thoracic and behind with (4) the two
sacral and (5) the first five caudals. Thus there is a long
group of fused vertebrae, known as the sacrum, to which
the pelvic girdle is attached. Then follow six free caudals
and the ploughshare bone or pygostyle, which consists of
four fused vertebrae and supports the tail. Each rib has a
head or capitulum which articulates with the centrum of
its vertebra and a tubercle which articulates with the
transverse process. Those which join the sternum are
bent forwards at an angle to do so, the part above the
angle being known as the vertebral rib, that below as the
sternal rib. Both parts are bony in the pigeon, whereas

5°°

MANUAL OF ELEMENTARY ZOOLOGY

in the rabbit the sternal ribs are cartilaginous. On the
hinder side of each of the free ribs, except those of the last
pair, is an uncinate process. The skull is remarkable
for the fusion of most of its bones. There is a short, wide
cranium, lying mainly behind the large orbits, which
are separated, not by the cranium, but by an interorbital
septum (p. 478). A scaffolding of slender jawbones
supports the beak.

The hinder part of the cranium is formed by two exoccipitals at the
sides of the foramen magnum, a median basioccipital below and a

median supraoccipital above. There is one median occipital condyle,
formed mainly by the basioccipital. The roof of the cranium in the
middle and foremost regions is formed by the parietals and frontals.
In the region of the parietals the floor is formed by the basisphenoid,
which lies in front of the basioccipital, but is covered in below by a
broad membrane bone, the basitemporal, which perhaps corresponds
to the crosspiece of the parasphenoid. The side of the skull in this
region is formed mainly by the squamosal, from which a zygomatic
process projects forwards, lying free. Below the squamosal the wall
is derived in front from the alisphenoid and behind from the bones of
the auditory capsule united with adjoining bones, but the limits of
none of these can be made out. In the frontal region, the cranial
cavity is greatly restricted by the presence of the interorbital septum,
over which, however, it extends forward somewhat. The septum is

THE PIGEON

501

formed by the union of mesethmoid with presphenoid and orbitosphenoid
elements to form a single plate of bone with a thickened ventral edge,
known as the rostrum, representing the blade of the parasphenoid.
The frontal sends downward a postorbital process. The lacrymal of
each side is a small, fiat, curved bone, in front of and above the
orbit. In the olfactory region, the nasals are a pair of thin bones
in the roof, before the frontals. Their fore edges are deeply notched
for the nostrils. The vomers (prevomers) of the pigeon are vestigial.
In the common fowl they are represented by a slender median rod in
front of the rostrum. In the upper jaw, the palatines are a pair of slender

pIG 279. — The skull and some of the cervical vertebrae of a pigeon,

from the left side.

d Dentary; Ku.. Eustachian tube; e.oc., exoccipital \ f.r., fenestral recess
’’ frontal; i.o.s.t interorbital septum; lac ., lacrymal; nasal ; par., parietal ,
f>l. palatine; pm., premaxilla; p.o.p ., postorbital process of frontal, pt .,
pterygoid; *., quadrate; s.o.b., suborbital . bar ; *.<*., supraoccipital , sa
supra angular ; sg., squamosal; zy., zygomatic process of the squamosal, / ,
//., foramina for first two cranial nerves ; 1-3, first three cervical vertebra:.

bars placed lengthwise in the roof of the mouth. From the hinder
end of each a short, stout pterygoid slopes outwards and back-
wards to join the quadrate, which is a strong, three- branched bone
articulated above with the squamosal, in front with the pterygoid,
and below with the lower jaw, whose suspensonum it forms, lhe
premaxilla of each side is a large, triradiate bone with the main part
directed forward and fused with its fellow to form the tip of the beak,
while two other processes pass back to join the two forward processes
of the nasal and thus enclose the nostril. The maxilla is a rod lying
inside the lower backward process of the premaxilla and projecting
backward beyond it. It gives off a plate of bone, the maxillopalatine

502

MANUAL OF ELEMENTARY ZOOLOGY

process, on its inner side. A slender splint, the jugal, joins it to a
third slip, the quadrato-jugal, which articulates with the outside of the
lower end of the quadrate. Thus there is formed a fine suborbita!
bar. In the slender lower jaw, articular, angular, supra-angular,
dentary, and splenial ele-
ments can be made out. pm.

There is a columella auris
and a slender, mainly bony,
forked hyoid apparatus.

The shoulder girdle
contains narrow, sabre-
like scapulae, stout
coracoids which slope
down to join the
sternum, and slender
clavicles which unite
to form the “ merry
thought.” Where
scapula, coracoid, and
clavicle join, a small
opening, the foramen
triosseum , is left be-
tween them. The
sternum is a broad
plate, bearing below a
conspicuous median
keel for the attach-
ment of the great wing
muscles, behind two
xiphoid processes, at
the sides facets for the

JX, x-

XII.

f. m.

Fig. 380. — The skull of a pigeon, seen
from below.

ribs, and in front sur- bt’> Basitemporal ; Eu., hinder opening of passage

for Eustachian tube ; Eu'., anterior opening of
the same foramen magnum; m., max-

illa; t>ip. , maxillopalatine process ; o.c.,
occipital condyle ; pi., palatine ; pm., pre-
maxilla ; pt., pterygoid; q., quadrate; q.j. ,
quadrato-jugal ; rs., rostrum ; IX., X,, XII.,
foramina for cranial nerves.

faces for the articula-
tion of the coracoid
bones. In the wing
skeleton there is a

short, stout humerus,
a parallel radius and ulna, rather widely separated except at
their ends, where they touch, only two free carpal bones,
those of the second row having fused with the metacarpals,
of which there are three, fused together, and three digits.
The thumb has one joint, the first finger two, and the

THE PIGEON

5° 3

second one. In the pelvic girdle there is a long ilium,
reaching a good way behind as well as in front of the
acetabulum, and connected with the sacrum along nearly
the whole of its inner side. This, together with the length
of the sacrum, enables the trunk to be supported in a

more or less horizontal position
by the single pair of legs. The
acetabulum is placed nea?
the middle of the ilium. The
ischium is a flat, backwardly
directed bone. Its hinder part
is fused with the ilium, but just
after the acetabulum an oval
opening— -the iliosciatic foramen
— lies between the two. The
pubis is slender and also directed
backwards. In many birds it
has a small prepubic process in
front. The obturator foramen
is slit-like. There is no sym-
physis or ventral junction of
the girdles. The hind-limb has
a short, stout femur, a long
tibia, a slender fibula, partly
joined to the tibia below, no
free tarsals, these bones being
fused to the tibia and meta-
tarsals, a single tarso-metatarsus
formed by the union of the
distal tarsals with the meta-
tarsals (except the small, free,
first metatarsal), and four toes,
each of several joints.

The most conspicuous part of
the muscular system is the great pectoral muscles . The
pectoralis major , arising from the sternum and

Arrangements. c'av’c^e5 inst:rl;ed on the under side of the
humerus, which it pulls downwards, thus raising
the bird and driving it forward by the wing-beat in flight.
The smaller pectoralis minor arises from the sternum above
the major and passes through the foramen triosseum and

Fig. 381. — The hyoid appar-
atus of a pigeon.

a c.y Anterior cornu ; b., body of the
hyoid; b.br.i, b.br.v, basi-
branchials ; bi/td., basihyoid ;
p.c., posterior corrtu.

5°4

MANUAL OF ELEMENTARY ZOOLOGY

over the shoulder to its insertion on the upper side of the
humerus, which it raises. The perching mechanism is
also interesting. The flexor tendons which curve the

//., Humerus ; R.} radius ; U., ulna ; r., radiale ; u., ulnare ; C., distal
carpals united to carpo-metacarpus ; CC., the whole carpal region *

M C.I., metacarpal of the thumb ; /., phalanx of the thumb ; MC.1I
second metacarpus ; II., second digit ; MC.III., third metacarpus •

III., third digit. F., femur; T.T., tibio-tarsus ; Fi., fibula; Pi.,
proximal tarsals united to lower end of tibia ; dt., distal tarsals united
to upper end of metatarsus, forming a tarso-metatarsus ( T.MT.)\

I . entire tarsal region ; MT.I., first metatarsal, free ; I. -IV., toes.’

toes round a branch are so arranged that they are
tightened by the bending of the metatarsus on the tibia
in perching, so that the bird does not fall even when
it is asleep.

THE PIGEON

505

The mouth has no teeth, no true palate (false roof, pp.

Alimentary 4887 539^’ like the rabbit s> large posterior

system. nares partly hidden by soft palatal folds, a single

opening for the Eustachian tubes, and a sharp-
pointed tongue. The glottis is not protected by an epiglottis
as in the rabbit. The gullet widens into a thin- walled crop , in
which the food is stored. From the crop the gullet continues
to the pore-stomach or proventriculus, a glandular part of

n.

A

o g- pr.

Fig. 383. — The position of organs in a bird. — After Selenka.

n., Nostrils; tr., trachea; cr., crop; h., heart ; st., sternum; fir.,
proventriculus; g., gizzard; c., caeca; fi., pygostyle ; fiv ., pelvic
girdle; k., kidney; lung.

the stomach, where the gastric juice is secreted. This is
followed by the gizzard, a lens-shaped chamber with very
thick muscular walls and a horny lining, where the food is
ground up by the aid of small stones which have been
swallowed. It Pes below the proventriculus, which opens
on its dorsal border, rather to the left side : on the right
side near the same epot is the opening of the duodenum.
This is a V-shaped loop, between whose limbs lies the
pancreas. The duct? of this gland are three, and all open

5°6

MANUAL OF ELEMENTARY ZOOLOGY

into the distal limb of the duodenum, two about the
middle of its length, and one, which is longer than the
others, near the end. There are two bile ducts, which
run from the large, bilobed liver and join the duodenum,
the wide left duct opening into the proximal limb and the
narrower right duct into the distal limb near the first two
pancreatic ducts. There is no gall bladder in the common
pigeon. The ileum is a much-coiled tube about two and
a half feet in length. The rectum is about an inch and
a half long. Its beginning is marked by a pair of small
rectal coeca ; behind it opens by an anus
into the cloaca. This has three regions
separated by shelves of the wall.

The first and largest is the copro -
dcEum into which the rectum opens,
the small middle division is the
urodcBum into which the urinary and
generative ducts open, the third,
larger, is the proctodceum ; upon
its dorsal surface there opens in the
young a glandular sac, the bursa TX

Fabricii , of unknown function.

The spleen is a small, red body,
attached to the right side of the pro-
ventriculus.

The glottis, behind the root of
the tongue, opens into
0rg»r»8tOiry the voiceless larynx, from
which the long trachea,
strengthened with bony rings, leads
back along the neck, lying at first below the gullet and
then at its left side. At the base of the neck it divides
into the two bronchi ; these run outwards and backwards
to the lungs, which lie against the dorsal walls of the
thorax covered with peritoneum below only. The hinder
end of the trachea is dilated and forms, with the beginnings
of the bronchi, the syrinx or organ of voice. Sound
is produced by the vibration of the membrana semilunaris ,
a delicate vertical fold of mucous membrane, extending
forwards from the angle between the bronchi. The
latter not only give off tubes which branch and form the

cloaca of a male
bird. — After Gadow.

cd., Upper region of cloaca
into which rectum
opens ; ud., median
region into which ureter
(u.) and vas deferens
(vd.) open from each
side; yd., posterior
region into which bursa
Fabricii ( B.F .) opens.

THE PIGEON

5°7

Fig. 385. — A pigeon opened from the ventral side to show the principal

organs in their natural positions.

O-th.s., Anterior thoracic air sac; ab.s., abdominal air sac; ao.c , aortic arch;
br.a., brachial artery; c.c., common carotid artery; cl. o. , cloacal opening;
cr.y crop; dm., duodenum; inn., innominate arteries; lung; lv., l<Tft
ventricle; lr., liver; wr.,* muscles concerned in the movements of the syrinx;
aes., oesophagus \p.nt., pectoralis major muscle ; p.th.s., posterior thoracic air sac ;
pent., pericardium (torn open); pcs., pancreas; pet. a., pectoral arteTy ; r.v.,
right ventricle; sc l.a., subclavian artery; thm., thymus; tra., trachea, Th«
gizzard is seen opposite the forceps which hold back the abdominal wall.

MANUAL OF ELEMENTARY ZOOLOGY

spongy lungs, but also pass right through these organs
and are connected with a system of large air sacs, of which
there are altogether nine, named, from behind forwards, the
abdominals, posterior thoracics, anterior thoracics, intercla-
vicular and cervicals. Certain of the air sacs are connected
with air spaces in the bones. This arrangement adds some-

FlG. 3S6. — The urogenital organs
of a female pigeon. — From
Thomson.

A'., Kidney (metanephroa) with three
lobes; u., ureter; cl.> cloaca; ov.,
ovary; od., oviduct; ft., funnel at
end of oviduct ; r. r. od. , rudimentary
right oviduct.

Fig. 3S7. — The urogenital
organs of a male pigeon.
— From Thomson.

T., Testes ; V. , base of inferior vena
cava ; S.K., suprarenal glands ;
K., kidneys with three lobes
(1, 2, 3); u.y ureter ; v.d., vas
deferens ; v.s ., seminal vesicle ;
cl., cloaca.

what to the lightness of the bird, but is probably of greater
importance in raising the efficiency of its respiration by
increasing the flow of the air through the bronchi. Respira-
tion is brought about mainly by active expiration ; it is
not due to a pumping in of air, as in the frog, or primarily
to active inspiration, as in the rabbit. The movements of
breathing consist in the rise and fall of the sternum, which

THE PIGEON

509

compresses and relaxes the air sacs and lungs. The air
passages in the lungs and air sacs of a bird are admirably
adapted to give the very efficient respiration which the
exertion of flight and the high temperature of the creature’s
blood require. The fine branches of the bronchi end, not,
as those of a mammal do, in minute sacs in which the air
is stagnant, but in a network of air capillaries through
which the air circulates. At inspiration, air is drawn
through main branches of the bronchi into the great air

hr.

Fig. 388. — A diagram of a lung and its air sacs in the pigeon.

a.c.%., Anterior thoracic sac; ab.s., abdominal sac; br., bronchus; cer.s.,
cervical sac ; icl.s., interclavicular sac ; l., lung ; p.th.s., posterior
thoracic sac ; r.br., recurrent bronchi ; tru., trachcea. The arrows
show the direction of the air currents.

sacs ; at expiration it is forced from these through
recurrent bronchi into the system of air capillaries, and
from the latter through the bronchi to the exterior.

The kidneys are metanephric (pp. 548, 636). They lie in
the back under the sacrum as a pair of three-
Excretory and lobed bodies. From the hinder lobe of each a
organs.UCt'Ve ureter runs back to the cloaca. There is no
bladder. Nitrogen is excreted as uric acid, not
urea. The urine is very concentrated and in the cloaca the
uric acid is precipitated and the water, with some salts, is
saved for the body by reabsorption as in the rectum of the
cockroach. The sexes are, of course, separate. The testes.

MANUAL OF ELEMENTAL Y ZOOLOGY

5TO

lie in front of the kidneys. From each of them the vas
deferens, corresponding to the Wolffian duct of the dogfish
and frog, runs back on the outer side of the ureter to end
in a small swelling or seminal vesicle which opens into the
cloaca. When it is full of ripe sperm the vas deferens is
slightly convoluted. There being no penis, the sperm is
passed in coition by the cloaca of the male being closely
apposed to that of the female. The adult pigeon has only
one ovary, that of the right side having atrophied early
in life. The right oviduct also atrophies, but a small
vestige remains attached to the cloaca. The ovary is
covered with follicles which contain ova in various stages
of ripeness. The oviduct is a wide, twisted tube, thin-
walled in front and thick behind, opening into the body
cavity by a long funnel just behind the ovary. When
the ova are ripe they are shed into the body cavity and
immediately caught by the opening of the oviduct. Each
ovum is a large, round, yellow body which becomes the
“ yolk ” of the egg (Fig. 487). It is a single gigantic
cell, so full of yolk that the protoplasm is practically
restricted to a small patch at one side, containing the
nucleus. It is fertilised in the thin region of the oviduct,
coated with white of egg in the first part of the thick region,
and provided with a double membrane and a porous chalky
shell in the hinder part. The eggs are hatched by the
warmth of the body of the parents, who sit upon them in
turns. I he young, which emerge after sixteen days,
are provided with a scanty yellow down and, unlike
young chickens, are at first quite helpless, with closed
eyelids. They are fed by their parents with a creamy
fluid known as “ pigeon’s milk ” formed by the break-
ing down of the epithelium of the crop. They are
fledged at the end of three weeks, and after a few days’
education in flight by their parents go out into the world
for themselves.

The blood has a temperature of 420 C., which is higher
than that of mammals. This fact is no doubt
00 e$se $. connectec| w;th the active life of the bird and
the rapid metabolism which it necessitates. We have
already seen how the respiratory organs provide the ample
supply of oxygen which such metabolism demands. The

Car.

Car.

Fig. 3S9. — The principal arteries of a pigeon. — From Thomson.

A.M Anterior mesenteric ; tBr., brachial ; C., caudal ; Car., carotid ; CL., cceliac •
D.A., dorsal aorta ; F., femoral ; IL., iliac ; L.A., left auricle ; L.V., left ven-
tricle ; P., pectoral; P.A., pulmonary artery; P.V., pulmonary vein; PM
posterior mesenteric; R., renals ; R.A., right auricle; R.V., right ventricle-’
Sc., sciatic.

511

512

MAX UAL OF ELEMENTARY ZOOLOGY

red corpuscles are oval and nucleated. The heart has four
chambers, two auricles and two ventricles, there being no
sinus venosus or conus arteriosus. The impure blocd
returned by the venae cavae to the right auricle passes into
the right ventricle through an opening guarded by a
muscular valve without chordae tendineae. It is then
driven by the pulmonary artery to the lungs, whence it
returns by the pulmonary veins to the left auricle, passing
thence through two membranous valves with chordae
tendineae to the left ventricle, by which it is driven into
the single aortic arch. The openings of the aorta and
pulmonary artery are guarded each by three semilunar
valves. The aortic arch bends over to the right side,
giving off at its apex right and left innominate arteries,
from each of which arise a carotid and a subclavian. The
latter is exceedingly short, breaking up immediately into
brachial and pectoral branches. The further course ot
the arteries is shown in the diagram on fig. 389. dhe
venous system is shown in Fig. 390. 'There are three
venae cavae, as in the frog. Each superior vena cava is
formed bv the union of a jugular, a brachial, and a pec-
toral. The jugulars anastomose under the base of the
skull. The inferior vena cava arises by the junction ot
two iliac veins in front of the kidney. Each iliac vein is
formed bv the union of a femoral, a renal and a big
hypogastric which passes upwards through the kidney.
Behind the kidneys the hypogastrics arise in the following
way. The little caudal vein forks into two branches, each
of which runs through one of the kidneys as a hypogastric.
Each hypogastric is much larger than the caudal of which
it is a branch, because at the bifurcation another vein, the
coccygeo-mesenteric from the cloaca and large intestine,
joins the caudal, and immediately after it has separated
from its fellow the hypogastric receives an internal iliac
vein. In its course through the kidney it receives several
small renal veins and a sciatic. There is practically no
renal portal system, though the femorals give a few small
branches to the kidneys. A hepatic portal system exists
as usual. A vein usually known as the epigastric
takes blood from the great omentum, or sheet of fat
which covers the abdominal viscera, to the left hepatic

THE PIGEON

5*3

Fig. 390 — The principal veins of a pigeon. — From Thomson.

Br., Brachial ; C., caudal ; C.M., coccygeo-mesenteric ; E.P., epigastric ; F.,
femoral; H.V., hepatic; Hyp., hypogastric; i.il., internal iliac; I.V., iliac;
I.V.C., inferior vena cava ; J ., jugular ; K., kidney ; L.A., left auricle ; L.V.,
left ventricle; P.A., pulmonary artery; P.V., pulmonary vein; R., renal;
R.A., right auricle ; R.V., right ventricle ; Sc., sciatic.

33

5H

MANUAL OF ELEMENTARY ZOOLOGY

vein. It represents the anterior abdominal vein of the
frog.

The cerebral hemispheres of the brain are large,
smooth, and rounded. The roofs of the lateral
Nervous ventricles are relatively thin, though nervous,

sensMJrgans. but the corpora striata are large ; with this
development is connected the elaborate but
stereotyped behaviour of birds. The olfactory lobes are

Fig. 391. — The brain of a pigeon. — From Thomson.

d> Dorsal, (2) ventral, and (3) side view. c., Cerebral hemi-
spheres ; cb., cerebellum; nt.o., medulla oblongata ; o.l., optic
lobes; olf., olfactory lobes ; s.c., spinal cord.

very small. The cerebellum and cerebrum meet over
the thalamencephalon, thrusting the round, hollow optic
lobes to the sides. The cerebellum is ridged transversely.
There are twelve cranial nerves, corresponding to those
of the rabbit (p. 563). The sense of smell is not well
developed. Hearing is acute, the labyrinth possessing
the organ known as the cochlea which was quite rudi-
mentary in the frog. Sight is very keen, and the eye is
remarkable for the presence of a vascular pigmented

THE PIGEON

515

organ, known as the pecten , which protrudes into the
vitreous humour from the “ blind spot ” where the optic
nerve enters.

Warm-blooded though they are, birds are more akin
to reptiles than to mammals. This is expressed in many

details of their anatomy — the structure and articulation
of the lower jaw, various other features of the
skull, the ankle joint, the organs of reproduc-
tion, the preponderance of the right systemic
arch, the nucleated red blood corpuscles, the scaly legs,
etc. An interesting link between birds and reptiles is

Birds and
Reptiles.

MANUAL OF ELEMENTARY ZOOLOGY

5i6

the extinct Archaeopteryx , which is known only by two
fossil specimens from the Upper Jurassic. This creature
was, as far as is known, a bird in all essential features,
but had, like a reptile, teeth, free fingers on the hand,
and a long flexible tail of many vertebrae.

CHAPTER XXIV

THE RABBIT

The Rabbit, Lepus cuniculus , is one of the animals that
have been introduced into Britain by man.
H*blt,‘ Its original home was in the countries at the

western end of the Mediterranean. 1 hence it has spread
or been carried by man throughout most of Europe and into
various other parts of the world, where its adaptability and
great fertility have enabled it to thrive to such -an -extent
that often, as notably in Australia, it has become a serious
nuisance. Its habits are well known. It is herbivorous, and
will eat a great variety of plants. It is gregarious, and digs
for itself burrows into which it retires to sleep or at the
approach of danger and to rear its young. On this account
it prefers districts where the soil is light and easily worked,
though it will live even in wet places if these bear
dense vegetation, in which it can form runs instead of
burrows. As befits its defencelessness, it is very wary, and
its habit of living in societies gives each individual a better
chance of receiving warning of the approach of an enemy.
Its custom of feeding chiefly at dusk has similar advantages
in enabling it to escape observation. It lives seven or eight
years and breeds four times, or oltener, in a year, beginning
to breed at six months old. As each litter contains from
five to eight young, its natural rate of reproduction is
enormous and enables it to pay the heavy toll taken by its
numerous enemies. It is readily domesticated, and various
fancy races have been produced by breeders.

The rabbit is covered with fur , which in the wild race
is of an inconspicuous, tawny-grey colour save
£*te™ai on the under side of the short, upright tail,
where it is white. When, on an alarm, the
animal scampers off to its burrow, the white patch on its

5i7

5 1 8 MANUAL OF ELEMENTAL Y ZOOLOG Y

tail is conspicuous, and this, though no doubt it enables an
enemy to follow the fugitive, has probably advantages to the
species in guiding and warning other members of the society.
The head is separated from the trunk by a distinct neck, a
feature which we have not met with in the dogfish or frog.
The long external ears or pinna are another new feature.
The eyes have movable upper and lower lids with a few
eyelashes , and a small third eyelid lies as a white membrane
in the inner corner and is used in cleaning the cornea.
This eyelid is rudimentary in man. The nostrils are two
oblique slits at the end of the snout, and lead internally
into the pharynx. We have seen that in the dogfish the
nostrils do not open internally and in the frog they open
into the front of the mouth. The upper lip is a “ hare
lip,” cleft in the middle, the cleft being continuous with
the nostrils and exposing the great front teeth. On the
sides of the snout and round the eyes there are strong
tactile hairs or vibrissa which correspond to the so-called
‘whiskers” of the cat. There is no cloaca , the anus and
urinogenital openings being separate, and the latter in front
of the former, in the male on the end of a penis, in the
female within a slit-like vulva which contains in front a
small clitoris corresponding to the penis. Beside the penis
in the male lie the scrotal sacs, into which the testes of the
adult descend, but there is no hanging scrotum. Along
the breast and belly of the female there are four or five
pairs of teats on which open the milk glands of the mamma,
which we meet now for the first time. At the sides of the
anus are a pair ot hairless depressions, into which open the
duets of the perineal glands , to whose secretion is due the
peculiar smell of the rabbit. The limbs have the same
general shape as those ot the frog and other land vertebrates,
being ol the type known as pentadactyle (pp. 47-49), though
in the rabbit, while the fore-limbs have five digits, the hind-
limbs have only four. The digits end in horny claws.
The fore-limbs are shorter than the hind-limbs, and in
running the animal does not tread upon the whole sole of
the foot, carrying the heel above the ground.

The closely related Common Hare differs from the
rabbit in its greater size, the greater length of the hind-
limb, the black tips of the very long ears, the absence of

52°

MANUAL OF ELEMENTARY ZOOLOGY

the burrowing habit, and the fact that the young, which are
Hares. born °Pen> are hairy, whereas those of the

rabbit, born in the shelter of a burrow, are naked.

1 he hare is a native of Britain and other parts of Northern
Europe. The Mountain Hare is more like the rabbit in
the shape of its body, but has black tips to the ears and
turns grey or white in cold weather.

The rabbit is a backboned animal, with all that we have
seen that to imply (p. 421). Its skin, like that of all Verte-

Fig. 394. A diagram of a section through the skin of a mammal.

Highly magnified. — From Shipley and MacBride.

kv., Blood vessels ; c.tiss connective tissue of dermis ; d.sw.g duct of sweat gland ;
der , dermis or corium ; efiid ., epidermis ; h.l., stratum corneum or horny layer
of the same ; hr., hair ; mus. , muscles by which the hair may be made to stand
on end ; JI.l., Malpighian layer ; p ip., hair papilla ; sb.g sebaceous gland ;
sw.g., sweat gland.

brata, is covered with a stratified epidermis. There are no
scales, but cellular outgrowths of the epidermis

Anatom and ^orm which are peculiar to the warm-

$kin * blooded, suckling animals known as Mammalia.

Each hair is embedded in a pit or follicle of the
epidermis, at the bottom of which it arises by the growth of
the epidermic cells which cover a vascular papilla. The
bristles of the crayfish or of hairy caterpillars, and the chsetae
of the earthworm, are not true hairs, but cuticular structures
secreted by the epidermis. The skin also contains sweat
or sudorific glands and grease or sebaceous glands which

THE RABBIT

521

secrete an oily substance into the hair follicles- The glands
and follicles are parts of the epiderrhis, but projevct"in wards
into the dermis. Below the latter is a layer of fatty tissue.
The muscles of the adult rabbit, as in the frog, show little
trace of the segmentation which they have in the early
stages of development. The general arrangement of the
internal organs resembles that of the frog, but a muscular
partition, the midriff or diaphragm , separates off from the

ep.c

Fig. 395. — A diagram of a transverse section through the thorax of a

rabbit.

a med ., Ventral part of mediastinum; ao., aorta; i.v.c ., inferior vena cava; l.v.,
left ventricle ; ces., oesophagus; p.c., pericardial cavity; p.med., dorsal part of
mediastinum; pm pericardium; r.l., right lung; r.pl. , right pleura; r.pl.c.
right pleural cavity; r.v. , right ventricle ; sp.c., spinal cord ; st., sternum ;
vertebra.

peritoneal cavity of the abdomen a chest or thorax in the
breast region, where lies the pericardium, with on each side
a pleui~al cavity , into which the lung of its side projects.
The lining of each pleural cavity is known as a pleura, and
of course: covers the lung as well as the inside of the thorax.
The heart in its pericardium does not lie free in the cavity
of the chest, as that of the frog does in the anterior part of
the pleuroperitoneal cavity, but is fastened to the dorsal and
ventral walls of the thorax by a double sheet of membrane,

522

MANUAL OF ELEMENTARY ZOOLOGY

each sheet forming the inner wall of a pleural cavity.
Between the sheets is a lymph-space known as the
mediastinum . In the dorsal part of this space lie the
aorta, certain other blood vessels, and the oesophagus ;
its middle part is quite filled by the pericardium, with
which its walls fuse ; and in its ventral part lies the
thymus.

The skeleton of the rabbit in its main features, and to
a considerable extent in its details, resembles
Backbone that of the frog, but only in its broadest out-
lines can a correspondence with that of the
dogfish be traced.

A B C

Fig. 396. — A diagram of the perivisceral coelom :
A , of the dogfish ; B , of the frog ; C, of the rabbit or man.

d., diaphragm ; h., heart ; lung ; p.c., peritoneal cavity ; pl.c.,
pleural cavity ; pi. p.c., pleuroperitoneal cavity.

Like those ot both the above-mentioned animals, it is composed of
an axial part, consisting of the skull, the backbone of vertebrae, and
the breastbone (sternum), which supports the head and trunk ;
and an appendicular part, comprising the bones of the limbs and their
girdles, which supports the limbs and anchors them to the trunk.
The plan upon which the parts of the appendicular skeleton of the
rabbit and other terrestrial vertebrate animals are built is shown
in Fig. 48. Departures in the rabbit from this scheme are com-
paratively unimportant. The most considerable of them is the loss
of most of the ventral region of the shoulder girdle. The structure of
joints is described on p. 50.

The skeleton is almost entirely bony, though most
of it is first laid down in cartilage, which persists
upon the surfaces of the joints and elsewhere. The

524

MANUAL OF ELEMENTARY ZOOLOGY

vertebrae1 are much like those of the frog . (p. 38),
each of them being entirely bony and consisting of
a body or centrum with two neural arches, which
enclose above the centrum a vertebral foramen, sur-
mounted by a neural spine or spinous process. As in
the Irog, each arch bears in front an upward-facing
facet or superior articular process or prezygapophysis and
behind a downward-facing inferior articular process or
postzygapophysis which fits on to the corresponding
prezygapophysis of the next vertebra, while at the side a
transverse process projects, and at each end there is an
intervertebral notch for the passage of a spinal nerve, the
adjacent notches of two vertebrae enclosing an inter-
vertebral foramen. Each end of each centrum, with the
exception of the first two, is flat, and against it in the
young rabbit is a thin bony disc cr epiphysis , which fuses
with it when growth is complete. There is more difference
between the vertebrae than in the frog, the backbone being
divided into five sections, the neck or cervical, chest or
thoracic, loin or lumbar, hip or sacral, and tail or caudal
regions. In the cervical region there are seven vertebrae,
which may be recognised by the fact that apparently each
of the transverse processes is pierced by an opening (its
foramen) : there is thus formed on each side a verte-
br arterial canal , through which pass the vertebral artery
and vein. This is due to the fusion with the vertebrae of
short cervical ribs in such a way as to constitute a com-
pound “ transverse process ” which encloses a space. The
first vertebra, known as the atlas , is ring-shaped, with a
very large vertebral foramen and no centrum. The ring is
divided by ligament into an upper part, through which the
spinal cord passes, and a lower part, into which fits a peg,
the dens or odontoid process , projecting forward from the
centrum of the second vertebra. This peg represents the
centrum of the atlas removed from it and fused with the
vertebra behind. The transverse processes of the atlas
are very broad, and the front side of the vertebra has two
very large articular surfaces' for the occipital condyles.
The second vertebra is known as the axis or epistropheus.

1 The general characters of the vertebrae of the rabbit may be well
studied in that known as the second lumbar (see below).

THE RABBIT

525

It has a long, crest-like neural spine and bears the
odontoid process. The remaining cervical vertebrae are
short and broad, with low neural spines, except that ol
the seventh. The thoracic region contains twelve or

n.s. b

Fig. 398. — Vertebrae of a rabbit.

A, Atlas, from above; B, axis, from the right; C, one of the middle cervical
vertebra, from in front ; D, fourth thoracic vertebra, from the right ; E, second
lumbar vertebra, from the right ; F, the same from in front.

aps., Anapophysis ; az., prezygapophysis ; cm., centrum ; c.r., cervical rib, fused to
transverse process and centrum ; ep., epiphysis ; /., facet on axis for articulation
with atlas ; /'., corresponding facet on atlas ; /"., facet on atlas for odontoid
process ; f.c., f.c'., demi-facets for heads of ribs ; f.t., facets for tuberculum ;
hps., hypapophysis ; mps., metapophysis ; n.a., neural arch ; n.s., neural spine ;
od.p., odontoid process ; pz., postzygapophysis ; tr., transverse process ; v.c..
vertebral foramen ; ver.c., foramen of transverse process.

See also Fig. 399, A and B.

526 MANUAL OF ELEMENTARY ZOOLOGY

thirteen vertebrae, which are characterised by bearing mov-
ably articulated ribs. The neural spines are tall, the trans-
verse processes short and stout, and each, in the first nine
vertebrae, provided on the under side with a facet or “ costal
pit ” for articulation with the tubercle of a rib, presently
to be described. The front end of the centrum (in the first
nine the hinder end also) bears on each side a facet for the
head of the rib. The hinder vertebrae of this set gradually
become more like those of the lumbar region. These are
usually seven in number. They are characterised by their
large size and the great development of their processes, the
prezygapophysis being borne upon the inner side of a large
metapophysis and the hinder intervertebral notch being
overhung by a small anapophysis. In the first two the
centrum bears a median ventral hypapophysis. The
lumbar vertebrae have no ribs. There is usually only one
sacral vertebra , but sometimes two are found. These
vertebrae are large and bear at the sides a pair of wing-like
expansions, which support the hip girdle and are probably
ribs fused with the vertebra. A certain number of the
succeeding vertebrae are fused with the true sacral vertebra,
the whole mass being known as the sacrum. The caudal
region contains about eighteen vertebrae, of which the first
three or four are fused with the sacral. They grow smaller
from before backwards, losing their processes and be-
coming degenerate.

The ribs are present as independent elements only in the
thoracic region. They are curved, bony rods,
Breastbone. articulated with the vertebrae. Those of the
first nine pairs are connected at their lower
ends with the breastbone by bars of calcified cartilage
known as their sternal portions or as sternal ribs. The end
which articulates with the vertebra has a knob known as the
head or capitulum. The first nine pairs have a second facet
on the dorsal side at a short distance beyond the head. This
is for articulation with the transverse process of the vertebra ;
immediately beyond it, for the attachment of ligaments,’
is a short projection, together with which it forms the
tuber culum. The sternal portions of the first seven pairs
articulate directly with the sternum ; those of the eighth and
ninth are connected with the ribs in front of them. The

THE RABBIT

527

last three pairs have no sternal portions and no tubercula.
The breastbone or sternum is a long, narrow rod, divided
into segments, and lying in the mid-ventral line of the
thorax. The first segment is the manubrium . It is the

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largest and is flattened from side to side. Behind it come
four segments of equal size, then a very short segment, and
finally the xiphoid process or xiphisternum , a long, slender
rod, which bears behind a horizontal plate of cartilage.
The ribs of the first pair articulate with the sides of the

528 MANUAL OF ELEMENTARY ZOOLOGY

manubrium, and the succeeding six pairs between the
segments.

The skull 1 contains the same regions that we have met
Sku„ with in the frog and dogfish, but it consists

practically entirely of bones, which meet one
another by jagged sutures.

The cranium or brain-case proper is relatively short, lies almost
wholly behind the orbits, and is not in a line with the facial region,
which is bent downwards at an angle of 6o° upon it. Its bones are-
arranged in a series of three rings, (i) The hinder or occipital ring
consists of four cartilage bones (p. 40). The basioccipital is a flat
bone which forms the floor of the ring, including the lower edge of the
foramen magnum and a small part of each occipital condyle. The
exoccipitals make the sides of the ring, bounding the foramen laterally
and forming the greater part of the condyles. The supraoccipital is a
large, median bone which roofs the occipital ring. (2) In the middle or
parietal ring there are both cartilage and membrane bones. It abuts on
the occipital ring above and below, but at the sides is separated from it
by the auditory capsules and squamosal bone. The floor of the cranium
in this region is formed by a cartilage bone known as the basisphenoid.
which lies in front of the basioccipital. It is triangular with the apex
truncated and placed forwards, and upon its upper surface is a hollow,
known as the sella turcica , which lodges the pituitary body. The
alisphenoids are a pair of irregular cartilage bones which lie at the sides-
of the basisphenoid and form the lower part of the lateral wall of the
cranium. The parietals are two large, square membrane bones upon
the roof of the cranium, separated at the sides from the alisphenoids by
the squamosals. The parietals meet in the middle line. Behind there-
is wedged in between them and the supraoccipital a small median
interparietal. (3) The foremost or frontal ring contains a narrow
median ventral cartilage bone known as the presphenoid, which lies in
front of the basisphenoid and is connected with it by cartilage. With
the presphenoid are fused at the sides a pair of cartilage bones known
as the orbitosphenoids , which form the lower part of the lateral walls,
of the cranium in the orbital region. Above them the frontals, a
pair of large, oblong, membrane bones, complete the side walls and
form the roof, each bearing a large supraorbital process. (4) The front
wall of the cranium is formed by a partition of cartilage bone, known
as the cribriform plate , pierced by a number of holes, through which
the olfactory nerves pass to the nasal capsules.

We have seen that the occipital and parietal rings are separated on
each side of the cranium by a gap, in which stand the auditory capsule

With the aid ot bigs. 44-6 and 447- the following account may be
applied to the skull of the dog, which is in some respects a more
suitable example than that of the rabbit for a preliminary study
ot the mammalian skull. The most important differences are in the
dentition (p. 583) and the conformation of the facial region.

THE RABBIT

529

and squamosal . The latter is a large membrane bone which abuts on
the parietal, frontal, alisphenoid, and orbitosphenoid. From its outer
surface there arises a stout zygomatic process, which bears on its under
side the fossa for the articulation of the lower jaw and, beyond the

Fig. 400. — The breastbone and shoulder girdle of a rabbit, seen from
below and somewhat from in front.

ncr., Acromion ; cl., clavicle ; cor., coracoid process; cp., capitulum ; g.c glenoic
cavity; mb.,, manubrium ; ntcr., metacromion ; sc., scapula; st. r. , sternal
portion of a rib ; st. 2, st. 6, second and sixth sternebrae ; tb., twberculum ; v.r.„
vertebra! portion of a rib ; x., xiphisternum ; x.c., xiphoid cartilage.

See also Fig. 399 B.

facet, bends downwards to join another bone, the jugal, presently to
be mentioned, thus forming the zygomatic arch or cheek bone (see
p. 481). From the hinder border of the squamosal a slender post-
tympa?iic process extends backwards. The auditory capsule consists
ot a large cartilage bone known as the periotic, which ossifies in
development from three centres, one of which represents the prootic.

34

530

MANUAL OF ELEMENTARY ZOOLOGY

This bone fits loosely into a gap between the squamosal and the
exoccipital. Its inner part is dense and known as th e, petrous portion .
this encloses the auditory labyrinth. The outer part, which shows on
the surface of the skull, is the mastoid portion. Against the lover
part of the periotic is placed a thin membrane bone snaped like, a
flask with a gap on one side, the gap being turned towards the periotic.
This is the tympanic bone. The body ot the flask, or bulla , encloses
the tympanic cavity, and the neck leads upwards and outwards from
the drum to the ear opening, the passage it encloses being known as
the meatus auditorius externus. At its inner end is a ring which
marks the position of the drum in life. The inner wall ot the tympanic

Fig. 401. — A diagram of the skull bones of a mammal (partly
after Flower and Weber), the membrane bones shaded.

S.Q., Basioccipital ; E.O., exoccipital ; C., condyle; S.O., supraoccipital ;
Pa**., parietal; Fr. , frontal ; N a., nasal; P >nx. , premaxilla , M.E.,
mesethmoid ; E., lachrymal ; Pu., turbinal ; P.S. , presphenoid ; O.S.,
orbitosphenoid ; A.S. , alisphenoid ; B.S., basisphenoid ; SQ., scpiamosal ,
P., periotic; T., tympanic*, PL, palatine; PL, pterygoid; Mx.,
maxilla; Ju., jugal; T.H., tympanohyal ; S.H., stylohyal ; E.M.,
epihyal ; C.H. , ceratohyal ; B.H., basihyal ; Th.H ., thyrohyal .
vomer ; MN., mandible.

cavity is formed by the periotic bone, and on it may be seen two gaps,
the fenestra avails and behind this the fenestra rotunda. In file a
chain of three little cartilage bones, the malleus , incus, and stapes,
connects the tenestra ovalis with the drum as the columella auns ot
the frog does, and like it transmits vibrations of sound (Fig. 427).
These bones belong in reality to the visceral arches.

The part of the skull in front of the cranium is known as the facial
region. It consists of the nasal capsules and certain of the bones of
the upper jaw, and we have seen that it is bent downwards at an angle
of 6o° with the cranium. The nasals are elongated membrane, bones
which form the roof of the nasal cavities, uniting by a suture with the
frontals behind. The mesethmoid is a median, vertical plate, ot
cartilage extending forward from the cribriform plate and separating

THE RABBIT

sloe-

Fig 402. — A ventral view of the skull of a rabbit.

crl.y External process of the alisphenoid ; b.oc.y basioccipital ; b.sp
basisphenoid ; e.a.m., external auditory meatus ; ex.oc., ex-
occipital ; f.m., foramen magnum; itic., incisors; ju.t jugal;
m r., molars; mx maxilla; oc., occipital condyle; pert
periotic; pi-y palatine; p*n-y premaxilla; ptttr., premolars.
pr.sp., presphenoid ; pt.y pterygoid; s.oc., supraoccipital ;
ty.b.y tympanic bulla ; v vomer ; zy.mx., zygomatic process
of maxilla ; zy.s., zygomatic process of squamosal.

532

MANUAL OF ELEMENTARY ZOOLOGY

the nasal cavities. The vomer is not comparable with the “ vomers ”
of the frog (prevomers). Its forepart is vertical and has on the upper
side a trough that encloses the lower edge of the mesethmoid cartilage
which supports the nasal septum (p. 539). Behind it sends out
“ wings ” towards the sides of the nasal cavity, so as to form a
horizontal partition, which separates an upper olfactory chamber
from a lower narial passage (p. 539). The outer sides and floor of
the nasal cavities are formed by the palatines, maxillae, and pre-
maxillae presently to be mentioned. The surface of the cavities is
increased by three pairs of thin and much-folded plates of cartilage
bone known as the twbinals which project into them from their

Fig. 403. — A side view 01 a rabbit’s skull. — From Thomson.

Pmx.y Premaxilla ; Na., nasal ; Fr., frontal ; Pa., parietal; Sq., squamosal ; S.O. ,
supraoccipital ; Per., periotic; T tympanic (the reference line points to the
bony external auditory meatus, beneatn it lies the in ated bulla) ; P. J., par-
occipital process*' of exoecipitali

walls. In the upper jaw there may be recognised the same two series
of bones as in the frog, the bones being membrane bones. The
pterygoids are two vertical plates of bone attached to the lower side
of the cranium at the junction of the basisphenoid with the alisphenoid
bones. The palatine bones are a larger pair, which consist each of a
vertical portion, attached above to the ventral side of the presphenoid
and behind to the pterygoid, and a horizontal portion which meets
its fellow in the median plane in the roof of the mouth (p. 539).
There is no quadrate bone. The premaxillee ( ossa incisiva) are a
pair of bones which form the front of the upper jaw and lodge the
upper pair of large gnawing teeth. It has a nasal process, which
passes backwards beside the nasal bone, and a palatine process,
which, like that of the palatine bone, forms part of the floor of the

THE RABBIT

533

nasal passages. The maxilla are two large irregular bones which he
behind the premaxillae in the facial region. The main part of each
bears the upper grinding teeth. From this arises a palatine process,
like those of the premaxillae and palatine bones, which it connects so
as to form a floor to the narial passages, and a zygomatic process,
which passes outwards and backwards to form the front part ot the
zygomatic arch. The zygomatic processes of the maxilla and squa-
mosal are joined by a bar of bone known as the jugal or malar bone
or zygoma. The lacrymals are a pair of small bones which torm
part of the front walls of the
orbits, lying between the frontals
and maxillae.

B.ky -0 ' i Tv

The lower jaw is composed
of membrane bone and repre-
sents the dentaries of the frog,

Meckel’s cartilage, which is
present during development,
being absent in the adult.

The jaw articulates, not with
a quadrate but with the

squamosal bone (see p. 478
and Fig. 405). The hyoid
bone , lying in the floor of the
hinder part of the mouth,

represents part of the visceral
skeleton. It consists of a
median body, representing the
basihyal, and two pairs ol
backwardly projecting cornua,
of which the hinder are the
larger. The anterior cornua
represent the hyoid arches,
and are completed by a series
of small separate bones, which

connect the hyoid bone with the periotic region 01^ the

skull. The posterior cornua represent the first pair of

branchial arches. The rest of the visceral skeleton is
represented by the jaws (mandibular arch), the ear ossicles
(hyomandibula, quadrate, and piece of mandible), and the
cartilages of the larynx (hinder branchial arches).

The following openings exist in the wall of each side of the skull :
(1) The anterior nares, at the front end of the nasal capsule, lor the
nostril. (2) The anterior and (3) the posterior palatine foramina , a

Fig. 404.— A dorsal view of a
rabbit's skull. — From Thom-
son.

S.O.y Top of supraoccipital ; //., in
terpanetal ; 7., tympanic; Pa.y
pai ietal ; squamosal; Pr.t

frontal; jugal; Na., nasal;
Pm. v., premaxilla.

534

MANUAL OF ELEMENTARY ZOOLOGY

Fig. 405. — A diagram of the
jaws of vertebrate animals.

A. The arrangement in the dogfish, i
which the jaws are not at any point
directly applied to the skull but are
suspended by the hyomandibular
which fits on to the skull (hyosivlic) ;

B, the arrangement in the' frog, in
which the lower jaw is suspended by
the quadrate, which is directly applied
to bones of the skull ( autostylic ) ;

C, the arrangement in the rabbit, in
which the lower jaw is suspended by
the squamosal.

c., Cranium; Ji., hyomandibular; lig.,
ligaments ; m . , lower jaw ; p.,

palatine ; pq., palatoquadrate bar ;
pt., pterygoid; q., quadrate; q?.,
one of the ear ossicles, possibly
representing the quadrate ; sq.,
squamosal.

In most fishes the jaws are hyostylic ;
in amphibians, reptiles, and birds they
are autostylic.

large and a small opening in the
palate for the passage of pala-
tine branches of the maxillary
nerve and blood vessels. (4) The
lacrymal foramen between the
lacrymal and maxillary bones, for
the lacrymal duct which drains
tears into the nose. (5) The
infraorbital foramen in front of
the zygomatic process of the
maxilla, tor the passage of a
branch of the maxillary nerve
from the orbit to the face. (6)
The optic foramen, a large round
hole in the orbitosphenoid for the
optic nerve. (7) The foramen
lacerum anterius or sphenoidal
fissure , a vertical slit between
the basisphenoid and alisphenoids
tor the third, fourth, sixth, and
ophthalmic and maxillary
branches of the fifth nerves. In
the dog and most mammals the
last-named branch passes through
a separate opening, the foramen
rotundum. (8) The foramen
lacerum medium , an irregular
opening on the under side of the
skull between the alisphenoid
and the periotic. Its anterior
part represents the foramen ovale
ot the dog and other mammals
and transmits the mandibular
branch of the fifth nerve. (9)
The stylomastoid foramen , a
small opening behind the tym-
panic, through which the seventh
nerve leaves the skull. (10) The
foramen lacerum posterius, an
irregular opening on the under
side ot the skull, between the
occipital condyle and the tym-
panic bulla, through which the
ninth, tenth, and eleventh nerves
and the internal jugular vein
pass. (1 1 ) The carotid foramen ,
which pierces the tympanic bone
near its inner border, close to
the occipital condyle, and trans-
mits the internal carotid artery.
(12) The condylar foramina , a

THE RABBIT

535

couple of holes in the exoccipital, just in front of the condyle,
through which the hypoglossal nerve passes in two divisions. Iq
connection with the tympanic cavity there are two openings, the
Eustachian canal at the anterior and inner angle of the tympanic
bone, on the under side of the skull, behind the foramen lacerum
medium, and the external auditory aperture at the end of the neck
or spout of the tympanic flask.

The shoulder girdle practically consists of one bone, the
Limbs. scapula, on each side. This is a flat, triangular

structure, with the apex downwards and
forwards, and bears a prominent external ridge or spine ,
which at its lower end becomes free as an acromion with
a long, backward metacromion. At the apex is the

shallow glenoid cavity for
the humerus, in front of
which a small hook or
coracoid process represents
the coracoid bone of the
frog. Along the convex
dorsal border lies a narrow
cartilaginous suprascapula.
The clavicle is a slender,
curved bone, lying in a
ligament between the acro-
mion and the sternum. In
mammals which move the
forearm freely, as in man,
it is well developed and
articulates with acromion an

Fig. 406. — The hyoid bone of
a rabbit, from above.

ax ., Base of the anterior cornu ; b.,
body ; px., posterior cornu.

sternum.

The hip girdle is large, and each of its halves is known
as an os innominatum or os coxce . With the sacrum it
forms a ring called the pelvis. In each os coxte may he
recognised a large dorsal ilium articulated with the sacrum,
a posterior ischium, and a smaller, ventral and anterior
pubis which unites with its fellow in a symphysis. The
ischium and pubis are separated by a large obturator
foramen , above and below which they meet. Above the
obturator foramen all three parts of the os innominatum
are continuous around the acetabulum, into which the
head of the femur fits.

The limbs contain the same bones as those of the
frog and other animals which have toes (Fig. 24). In

536 MANUAL OF ELEMENTARY ZOOLOGY

the fore-limb the humerus has in front of the head a
bicipital groove for the tendon of the biceps muscle,
bounded by two roughened projections, on the inner side
the lesser tuberosity or small tubercle , and on the outer side
the greater tuberosity or large tubercle. At the lower end
is a pulley-like trochlea, above which are two supratrochlear
f os see, the coronoid fossa in front and the olecranon fossa
behind, a supratroch-
lear foramen putting
the two into communi-
cation. In the forearm
the radius and ulna are
distinct but not mov-
able upon one another,
the radius lying in front
of the ulna. In man
the lower end of the
radius rotates round
the ulna, so that the
former lies in front of
and obliquely across the
latter when the palm
faces downwards, but
parallel with and out-
side it when the palm
is turned upwards.

The position in which
the palm is downwards
is known as pronation ,
that in which it is up-
wards as supination.

In the frog the limb is

— ac.

-Ob.f

--IS.

ac.

Fig. 407. —The pelvic girdie of a
rabbit, from beneath.

Acetabulum; il., ilium; is., ischium;
ob.f., obturator foramen ; pu pubis ; sym.,
symphysis pubis.

fixed half-way towards pronation ; in the rabbit it is fixed
in the prone position. A large olecranon process of the ulna
fits into the olecranon fossa. In the wrist all the nine bones
of the typical plan are present, arranged, as usual, in a
proximal and a distal row with a central bo?ie or centrale
between them. In the proximal row of three bones the
radial is known as the scaphoid or navicular , the intermedi-
ate as the semilunar or lunate , and the ulnar as the
cuneiform or os triquetrum. In the distal row there are

THE RABBIT

537

four distal carpals, the first on the inner side being known
as the trapezium or greater multangular , the second as
the trapezoid or lesser multangular , the third as the os
magnum or capitatum , and the fourth, which represents

Fig. 40S. — Bones of the left fore-limb of man.

A , In pronation ; B, in supination.

• h Os multangulare majus or trapezium ; c.2, multangulare minus or trapezoid ; cA,
capitatum or magnum ; cA, hamatum or unciform ; cap.h., capitulum of the
humerus, with which the radius articulates ; cap.r., capitulum of the radius ;
cap.u., capitulum of the ulna ; cor./., coronoid, fossa ; hu,, humerus ; im.,
os lunare or semilunar ; me. 1, me A, first and fifth metacarpals ; pi., pistoform ;
r.c., os naviculare or scaphoid ; ra., radius ; ra.f., radial fossa ; st.p.r., styloid
process of the radius ; tro., trochlea ; u.c., os triquetrum or cuneiform ; ul.,
ulna.

538 MANUAL OF ELEMENTAL V ZOOLOGY

two fused, as the unciform or os hamatum. On the hinder
side of the wrist is a small pisiform bone. There are five
digits, of which the first is the shortest and the third the
longest. In the hind-limb, the femur has a prominent
head, below which are three rough prominences, the greater

Fig. 409.— The skeleton of the left fore and hind feet of a rabbit.

A, Fore-foot ; B, hind-foot.

Astragalus; c.i, first distal carpal or trapezium; c. 2, second distal carpal or
trapezoid; c. 3, third distal carpal or magnum; c. 4, 5, fused fourth and fifth
distal carpals or unciform ; ce., centrale ; ce., centrale of hind-foot or navicular;
cm., fibulare or calcaneum ; im., intermedium or semilunar; me., metacarpals ;
met., metatarsals ; ph., phalanges; ra., lower end of radius with its epiphysis;
r.c., radiale or scaphoid ; /. 2, second distal tarsal or mesocuneiform ; t. 3, third
distal tarsal or ectocuneiform ; t.\, 5, fused fourth and fifth distal tarsals or cuboid. ;
u.c.. ulnare or cuneiform ; ul. , lower end of ulnar with its epiphysis ; I.-V., digits!

THE RABBIT

539

trochanter on the outside, the lesser trochanter on the inner
side, and the third trochanter below the great trochanter.
At the lower end of the bone are two large condyles for the
tibia. A knee-cap or patella covers the knee joint and is
connected by ligament with the tibia. The tibia and fibula
are fused at their lower ends only. The latter is a small
splint of bone outside the former, which is straight and
stout and bears in front a prominent cnemial crest. In the
ankle the bones, like those of the wrist, are arranged in two
rows with a central bone between them. The first row
contains, as in the frog, two bones, the astragalus or talus,
which corresponds to a fused tibiale and intermedium, and
the fibulare or calcaneum, which lies outside the astragalus
and projects backwards to form the heel. The central
bone is known as the tiavicular. The distal row consists
of three bones, that which corresponds to the missing first
digit being absent, and those which correspond to the
outer two digits being fused together. The innermost of
the remaining bones of the row is known as the meso-
cuneiform , the next as the ectocuneiform , and the third as
the cuboid. The metatarsals are long and there are four
digits.

The mouth differs from that of the frog in the possession
of mobile, muscular lips, and of a palate — an
Alimentary inner roof which separates from the mouth a
Mouth! Teeth, narial passage. By this passage the approach
and Pharynx.’ from the nostrils to the mouth is prolonged
backwards, so that the internal nares open
into the pharynx instead of into the forepart of the mouth
(Fig. 410), The first part of the inner roof is strengthened
by the horizontal processes of the premaxillary, maxillary,
and palatine bones (p. 532) and is known as the hard
palate ; the hinder part is purely fleshy and is known
as the soft palate. The narial passage lies above the palate
and below the true olfactory chambers. Over the hard
palate it is not separated from these by any roof, and
the nasal septum between them comes down to divide
it into two (p. S32)- Over the soft palate it is single, and
is separated from the olfactory chambers by a partition,
supported by the horizontal flanges of the vomer, repre-
senting the true roof of the mouth. Into this hinder part,

540 MANUAL OF ELEMENTARY ZOOLOGY

the naso-flharynx, open the Eustachian tubes. The
tonsils are a pair of pits at the sides of the soft palate
near its hinder border. I he tongue is an elongate, mus-
cular mass attached along most of its length to the floor

c. c. bs

mn

Fig. 410.*— A vertical section through a rabbit’s head. — From Thomson.

pmx, Premaxilla with incisors; m.e., part of mesethmoid in front region, where
nari*il passage is not separate from olfactory chamber ; m.e'., part of same in
hinder region, where it divides from one another only the two olfactory
chambers, which are here separated by a horizontal partition from the single
narial passage (the intermediate part of the mesethmoid is cut away); t.b .,
maxillary turbinals ; e.t ., ethmoidal turbinal ; olf.l. , olfactory lobe of cerebrum ;
/>s., presphenoid ; c.c., position of corpus callosum ; 6s.-, basisphenoid with
depression for pituitary body ; cb., cerebellum ; b.o., basioccipital ; s.c., spinal
cord; n.p., narial passage; g., gullet; tr., trachea; epg., epiglottis; smx.,
submaxillary salivary gland ; s.l., sublingual salivary gland ; 1\, tongue ; pi.,
transverse portion of palatine ; mn., anterior end of mandible.

I of the mouth, but with a free tip in front. It bears
papillae of several kinds which subserve the sense of
taste. The teeth differ from those of the dogfish and frog
in that (i) they are not all alike, (2) they are inserted in
sockets in the jaw, whereas those of the dogfish are

THE RABBIT

54i

embedded in the skin and those of the frog are fused to
the jaw, (3) they are borne on the edges of the jaws only,
and not on the roof of the mouth like the vomerine teeth
of the frog, (4) instead of being continually replaced by
the upgrowth of the skin from a groove as in the dogfish, or
one by one as in the frog, they are in two definite sets, the
milk teeth and the permatie?it teeth , of which the first is lost at
an early age and replaced for life by the second. Unlike
the teeth of most mammalia, those of the rabbit do not,
when they have reached a certain size, narrow at their roots
so as to form fangs and cease to grow, but continue to be
added to below as fast as they are ground down at the top.
The teeth do not form a continuous series as in man, but
between the cutting-off teeth ( incisors ) in front and the
grinders in each cheek is a wide gap ( diastema ) where most
mammals have the holding ( canine or dog-teeth), which,
and others, the rabbit lacks. The upper jaw has two pairs
of incisors, the first pair long, curved, and chisel-shaped,
the second small and hidden behind the first ; and six
pairs of cheek teeth, which, like those of most herbivores,
have broad, ridged crowns. The cheek teeth are much
alike but are divided into two sets by the fact that the first
three pairs, known as premolars , are preceded by milk
teeth, while the rest, known as molars , are not. In the
lower jaw there is only one pair of incisors ; these are
shaped like the first pair above, with which they work to
gnaw off the food, which is munched fine by the grinders.
This jaw has two pairs of premolars and three pairs
of molars. It is usual to express the number and arrange-
ment of the teeth of mammals by a dental formula .
Thus, in the pig, which has a typical set of teeth,

the formula is i — c — pm - m — , giving 22 on each side

3 1 d 3

of the mouth, or 44 in all. With this we may compare

3 3

m

3

total for both sides being 28. Four pairs of salivary
glands , which are not found in the frog or dogfish, pour
their secretion into the mouth. The parotid gland on
each side lies behind the angle of the jaw, the submaxil-
lary gland lies against its fellow between the angles of

the dentition of the rabbit, which is i — c - pm - m the

1 o z 2

542

MANUAL OF ELEMENTARY ZOOLOGY

Fig- 411- The body of a female rabbit with the abdomen opened,
the organs being somewhat displaced so as to display them.

bl., Bladder; cm, casern m ; co., colon; F.t., Fallopian tube; f.o., fimbriated
opening of the oviduct ; im., ileum ; lr., liver ; ov., ovary ; rm., rectum • st.

; ur'» ureter ; Mi., r^ht uterus ; vag., vagina ; ,r.c., xiphoid cart i'l age!
Note also : regions of body (head, neck, chest, abdomen, tail), mouth, nostrils
hare lip, prominent incisor teeth, vibrissae.

THE RABBIT

543

the jaw, the infraorbital gland lies below the eye behind
the cheek-bone, the sublingual gland lies along the
inside of the mandible. The saliva moistens the food and
contains an enzyme, known as ptyalin , which turns starch
into sugar. The pharynx receives in front the narial passage
and the mouth. Behind, it leads above (dorsally) into the
gullet and below into the glottis, which lies shortly behind
the tongue, covered by a flap, known as the epiglottis,

which is stiffened by a carti-
lage. Thus in the pharynx
there cross one another the
passages by which the food
passes to the alimentary canal
and the air to the lungs. In
swallowing, the soft palate is
raised and thus closes the
posterior nares, while the epi-
glottis protects the opening
of the windpipe, so that when
the food is thrust backwards
by the muscles of the tongue
and pharynx it passes only
into the oesophagus. That
tube, which is
longer and nar-
rower than those
Fig. 412. — The duodenum of a of the frog and dogfish, runs
rabbit.— From Krause, in part backwards through the neck
after Claude Bernard. and cpestj above the trachea.

p., Pyloric end of stomach; g.b gall Shortly after passing

bladder with bile duct and hepatic . i A j* i _ . 1

ducts ; pancreatic duct. through the diaphragm, the

oesophagus joins the stomach.

This is a wide sac, placed athwart the body cavity and

wider at the left or cardiac end than at the right or pyloric

end ; it is curved, with the concave side turned forwards,

and the oesophagus enters at the bottom ot the concavity.

The pyloric end communicates with the intestine by a small

opening, the pylorus, provided with a sphincter. The small

intestine is a narrow, much-coiled tube, seven or eight feet

in length. Its first section or duodenum runs from the pylorus

along the right side of the abdomen nearly to the hinder

Stomach and
Intestine.

544 MANUAL OF ELEMENTARY ZOOLOGY

end of the latter and then turns forward, forming a loop.
In the mesentery between the two limbs of the loop lies
the thin, diffuse pancreas, whose duct enters the returning
limb of the loop about three inches behind the bend. The
liver is a large, dark-red, lobed organ slung from the
diaphragm by the falciform ligament ; in a groove upon
its right central lobe lies the elongated, dark-green gall

bladder, from which the bile duct runs backwards to open
into the dorsal side of the duodenum shortly beyond the
pylorus. The remainder of the small intestine is the
I eum ; it ends in a round swelling known as the saceulus
rotundus. The lining of the small intestine is beset with
numerous minute processes or villi , by which its surface is
increased. At the junction of the small and large intestine
is placed a very large tube, the blind gut or ccecum , marked

V

U
smx

p c.

t. m

J.c.

lar

XII.— hy

r.d.

■c.g.

inn.

ao.a.

s.v.c.

r.au.

pul. a.
r.v.

du ar.
l.au.

Ll.

l.phr.

l.pl.c.

F(G. 414.— A dissection of the neck and thorax of a rabbit. The heart
has been displaced a little to the right, and the pericardium
and thymus removed.

ao.a.. Aortic arch ; c.c., common carotid arteries ; c.sy., cervical sympathetic nerve ;
d.ao., dorsal aorta; dep ., depressor nerve; di., diaphragm; du.ar., ductus
arteriosus ; ex.j., external jugular vein ; f.c., point at which the common carotid
divides ; hy., hypoglossal nerve ; i.c.g., inferior or posterior cervical. sympathetic
ganglion; inn., innominate artery; i.v.c., inferior vena cava, lying in medi-
astinum; l.au., left auricle; l.l.y left lung; l.phr., left phrenic nerve; l.pl.c.,
left pleural cavity; l.v., left ventricle; lar., larynx; oes., oesophagus in neck;
ass'., the same in mediastinum ; p.c., posterior cornu of the hyoid ; ptil.a., pulmon-
ary artery ; pul.v., pulmonary vein ; r.au., right auricle ; r.d., ramus descendens ;

r. l. , right lung, one part bulging into mediastinum ; r.lar ., recurrent laryngeal
nerve; r.pl.c., right pleural cavity; r.v., right ventricle; s.c.g., superior
cervical sympathetic ganglion ; s.lar., superior laryngeal branch of vagus ;

s. v.c. , superior vena cava ; scl., subclavian artery and vein ; smx., submaxillary
gland; t.m., tendon of mandibular muscle; thy., thyroid gland ; tra., trachea;
v.g., vagus ganglion ; vag., vagus ; W. d., duct of submaxillary gland (Wharton’s
duct); X. , XII., cranial nerves.

tra . as*,

"S.lar.

— vag.
c.sy.
dep.
r. lar.

■g-

l.v.

pul.v.

r.pl.c.

i.v.c.

d.ao.

CCS

ex.j.

c.c.

set.

546

MANUAL OF ELF MENTAL Y ZOOLOGY

by a spiral constriction and ending blindly in a small,
finger-like vermiform appendix. The sacculus rotundus
opens into the caecum about an inch from the end opposite
to the vermiform appendix ; at the same end the large
intestine starts. The caecum is usually large in herbi-
vorous mammals. In it the cellulose walls of plant cells
are digested by bacteria which turn them for their own
purposes into sugar, and thus make them available for
the mammal. We have here an example of symbiosis
(p. 217). Two regions may be recognised in the large
intestine. The colon , which is not present in the frog or
the dogfish, is a sacculated tube about a foot and a half in
length where the greater part of the water in the intestinal
contents is salved by being reabsorbed ; the rectum is a
narrower tube about two and a half feet long, in which
fsecal pellets can be seen. The digestion (p. 9) of the
food of the rabbit resembles in general that of the frog
(p. 61), but is complicated by the preparatory process in
the mouth and by the conversion of cellulose in the caecum.
To provide tor these, the alimentary canal possesses, as
we have seen, features which are not found in the frog or
dogfish.

The spleen is a narrow, crescentic, dark-red body lying
close against the convex side of the stomach.
Glands? The thymus is a soft, pink mass in the media-
stinal space at the front of the thorax. The
thyroid is a thin, red body consisting of two lobes, one at
each side of the larynx, joined by a band across the ventral
side of the latter. Adrenals and a pituitary body (pp. 548.
562) are present. (On ductless glands, see pp. 21, 63.)

For its active life and warm blood the rabbit needs a
more elaborate respiratory apparatus than the
Oreg5nsat0ry lr°g* The chest or thorax is a closed box
whose side walls are formed by the ribs with
the muscles between them, and its hinder wall by the
diaphragm, which divides the main or pleuroperitoneal
coelom, parting two pleural cavities in front from a
peritoneal cavity behind (p. 521). The windpipe comprises,
besides the larynx, a long tube with rings of cartilage in its
wall. This is the trachea or windpipe proper, which leads
back along the neck and in the thorax divides into two

THE RABBIT

547

bronchi which join the lungs. These are not mere sacks,
like those of the frog, but spongy, because the bronchi
break up into numerous bronchioles , which end in minute
air sacs. The cavity of the thorax is enlarged from back to
breast by an outward movement of the ribs and from head
to tail by the movement of the diaphragm, which at rest is

Fig. 415. — Diagrams of the male genital and urinary apparatus of :

A, The frog ; B, the dogfish ; C, the rabbit.

The animal is lying on its back, the organs of the right side are shown, and the
bladder is turned to the left.

bl., Bladder ; cl., cloaca; kidney ; k'., anterior region of the same, which dis-
charges through the Wolffian duct (v.d.) ; k’ ., posterior region, which dis-
charges through a duct of its own, the ureter (ur.) ; in the frog this region is
not developed, though it is present in the newt. In the dogfish, the two
regions remain continuous. In the rabbit, regions roughly corresponding
to those of the dogfish become in the adult the “ epididymis ” (meso-
nephros) and “ kidney ” (metanephros) respectively. The “ pronephros ”
or foremost region of the kidney of the embryo has in each case disappeared
in the adult. Its history in the frog is described on pp.635, 636; t., Testis
(that of the rabbit is not lettered) ; ur., ureter ; v.d., vas deferens (Wolffian
duct) ; v.eff., vasa efferentia.

The arrangement in reptiles is intermediate between types B and C, the meso-
nephros being reduced to an epididymis, though it and the testis remain
in their original position, anterior to the metanephros.

convex towards the chest, but when it contracts flattens,
thus increasing the size of the thorax. Since the pleural
cavities are closed, their enlargement tends to set up a
vacuum within them, and thus the lungs, which are not
closed, expand to keep them full, drawing in air 1 through
the glottis. When the inspiratory muscles relax air is

1 That is, the air enters by its own pressure and expands the lungs
when the pressure around them in the pleural cavity is lowered.

548 MANUAL OF ELEMENTARY ZOOLOGY

I

driven out by the collapse of the chest owing to the
elasticity of the lungs, ; but this can be aided by the con-
traction of certain other muscles, notably those of the
belly, which press the viscera against the diaphragm
from behind, (For respiratory regulation, see p. 21).

The kidneys of the rabbit are a pair of dark-red bodies,
convex on the outer side and concave on the
Excretory and inner, which lie on the dorsal wall of the
organs.UCtlve peritoneal cavity, that on the left side farther

back than that on the right. Like those of
the dogfish and frog they consist of tubules, but these
even in the embryo have no coelomic funnels. The tubules
resemble those of the frog (p. 79) but each has a long
additional hank known as the loop of Henle . As the
filtrate from the glomerulus passes down the tubule,
water is reabsorbed from it, possibly in the loop of Henle,
and so is saved for the use of the body. The principal
nitrogenous substance in the urine is urea (see p. 80).
It will be recalled that, whereas the kidney of the frog has
no distinction of regions, and discharges solely by the
Wolffian duct, that of the dogfish (like that of the newt)
has a narrow anterior region, which takes little part in the
secretion of the urine, and a larger hinder region, which
is the main urinary organ and in the male possesses a
duct of its own, the ureter. In the embryo of the rabbit
a somewhat similar condition is found. A strip of kidney
tissue — the mesonephros — lying in front of that which
becomes the kidney of the adult, is served by a Wolffian
duct. In the adult male the mesonephros becomes
attached to the testis and the Wolffian duct becomes the
vas deferens. In the female these structures are aborted.
The adult kidneys of both sexes represent only the meta-
nephros or hinder part of the embryonic kidney, which,
instead of discharging through the Wolffian duct, has a
duct of its own, the ureter. From the concavity or hilus
the ureter runs back to open into the bladder. In the
early stages of development this organ joins the rectum
in a cloaca, but later the latter becomes divided, so that
the urinary and generative organs discharge by an
independent passage through the vulva or the penis.
Median to the kidneys lie two small, yellow, adrenal

THE RABBIT

549

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55°

MA NU A L OF EL EM E NT AR Y ZOOLOGY

glands. I he testes are a pair of ovoid bodies which arise
in the course of development on the dorsal wall of the
peritoneal cavity near the kidney, but later becomes free
and pass backward into two pouches of the body-wall at the
sides of the penis known as the scrotal sacs. Each testis
remains connected with its original position by a spermatic

bl.

Fig 417. —The reproductive organs of the rabbit. A, Male; B
remale. In each case the dissection is made from the left side!
tne animal lying on its back.

Hiflllic1' ’ riCaVr corp,us cavernosum ; c.cav.cl., corpus cavernosum of the

dvmi ’p?'F^°W?er! ?lan<f; e£d-> cauda epididymis; epd'., caput epidi-
dymis, t.t. Fallopian tube; f.o., fimbriated opening of the same ; ov. ovary-

p.g; perineal gland ; pn., penis ; pr., prostate ; r.g., rectal gland ; rm., rectum ;
sc.s scrotal sac ; sp.c., spermatic cord (cut short) ; sym., svmphvsis pubis ; t.
testis; ur ureter; ut., uterus; ut.m., uterus masculinus ; t ith., urethra •
v.a., vas deferens; vag., vagina; vest., vestibule.

cord, which consists of connective tissue with an artery
and vein. In passing backwards it carries with it the
mesonephros, which m the adult mav be seen as the
epididymis , lying along the side of the testis and enlarged
at the tront and hind ejids into a caput and cauda respect-
bely. 1 he cauda epididymis is connected to the scrotal
sac b> a short, elastic cord known as the yuberuaculum .

THE RABBIT

55i

Each epididymis consists of a mass of twisted tubules
joining into a single, much-coiled tube which becomes
continuous at the cauda with the vas deferens (or ductus
deferens). This passes forwards out of the scrotal sac,
curves over the ureter, and passes backwards again to open
with the mouth of a small median sac known as the uterus
masculinus, which lies above the neck of the bladder within
the pelvic girdle. The uterus masculinus opens into the
neck of the bladder, wdiich is known after their junction as
the urino genital canal or urethra , and passes backwards-
into the penis, at the end of which it opens. Beside the
uterus masculinus lie the prostate glands which pass their
secretion into the urethra, and behind the prostate are
Cowper’s glands. The penis is situated behind the sym-
physis pubis and in front of the anus. It has spongy,
vascular walls and is invested by a loose sheath of skin, the
foreskin or prepuce. The ovaries are small, oval bodies
attached behind the kidneys to the dorsal abdominal wall,
and show on their surface little blister-like projections,
known as Graafian follicles , each of which contains a
microscopic ovum. The oviducts open into the abdominal
cavity by wide, funnel-shaped fimbriated openings just
outside the ovaries. When the ova are ripe the follicles
burst and discharge the ova into the funnels, which at that
time extend over them. The first section of each duct is
narrow and gently sinuous and is known as the Fallopian
tube. It runs backwards and enlarges into the uterus , a
vascular-walled structure which joins its fellow in the
middle line anteriorly to the bladder to form the vagina.
This passes backwards within the pelvic girdle above the
neck of the bladder, with which it presently unites to form
the urino genital canal or vestibule , which opens at the
vulva. On its ventral wall lies the small, rod-like clitoris
and on the dorsal wall two small Cowper’s glands.

In most mammals the ripening of ova and their dis-
charge, or ovulation , takes place at fixed intervals, and
coition, usually, at the same periods. In the rabbit,
however, ova are not actually discharged except after
coition. The spermatozoa travel up the oviducts and
fertilisation takes place at the upper ends of the latter. The
ova pass down the oviducts, in which they segment. At

552 MANUAL OF ELEMENTARY ZOOLOGY

the end of the third day they reach the uterus. Here at
first they lie free. On the eighth day, however, they begin
to become attached to the uterine wall, and in the course
ot the next few days there is formed in connection with

Fig. 418. — The heart of a rabbit, seen from the right side,
alter the removal of the outer wall of the right auricle
and ventricle.

ao.a., aortic arch; ch.t., chordae tendinea; ; col.c., column® carneae ;

/ ov., fossa ovalis, the site of an opening through which in the
embryo there passed into the left auricle blood returning to the
neart by tne inferior vena cava, muca of which came from the
placenta and was therefore arterial (p. 66^ • this blood was directed
by a fold of the auricular wall known as the Eustachian valve, lying
between the openings of the left superior vena cava and the inferior
cava, traces of which fold remain in the adult heart ; i.v.c., inferior
vena cava ; i.v c' internal opening of the same ; l.s.v.c'., internal
opening of left superior vena cava; tn.p., muscufi papillares;
pul. a., pulmonary artery, cut open; r.au., wall of right auricle;
r.s.v.c., right superior vena cava ; r.s.v.c'., internal opening of the
same ; si., semilunar valve ; tr.v., tricuspid valve.

each of them a special organ, known as the placenta ,
in which blood vessels derived from the mother and the
developing young are in very close and extensive contact.
Through the thin walls of the two sets of blood vessels
interchange of fluid and gaseous contents takes place, and

THE RABBIT

553

in this way the nutrition and respiration of the young is
provided for until birth, which takes place at the end of
a month from fertilisation. Animals in which, as in the
rabbit, a great part of development takes place within the
body of the mother, so that the young when they are born
are beyond the need ol a shell or similar covering, are said
to be viviparous.

The heart of the rabbit lies in the Iront part of the chest,
enclosed in the thin pericardium, immediately behind the

Fig. 419. — A diagram of the heart ot the rabbit, in ventral view.

Lettering as in Figs. 414 and 418.

Parts containing venous blood are shaded.

soft, pink thymus. It has no sinus venosus or conus arteri-
osus, but there are two ventricles as well as
Blood Vessels : tw0 auricles (atria), so that four chambers
Heart‘ are present. Three venae cavae corresponding

to those of the frog open directly into the right auricle
(Fig. 418), and two pulmonary veins lead by a common
opening into the left auricle, the opening from the right
auricle into the right ventricle is guarded by a threefold
tricuspid valve (Fig. 418), with chordae tendineae, and a
similar twofold mitral valve guards the opening between
the chambers of the left side. The two sides do not com-
municate with one another, from the Iront end of the

554

MANUAL OF ELEMENTARY ZOOLOGY

right ventricle arises the pulmonary artery, and from the
front of the left ventricle, above the pulmonary artery,
arises the single aortic arch. The opening of’ each of
these vessels is provided with three semilunar valves. The
pulmonary artery divides to supply the two lungs, and the
arteries to the head and arms arise from the arch of the

aorta, which afterwards sup-
plies the trunk. In the beat-
ing ot the heart, the auricles
contract simultaneously, and
the ventricles follow immedi-
ately afterwards ; then after

a short pause the auricles

«»

I IG. 420. — I he circulatory system
ot the rabbit. — From Thomson.

(<*) Letters to right —

e.c. External carotid artery.
i.c. Internal carotid artery.
e.j External jugular vein.
scl.a. Subclavian artery.
scl.v. Subclavian vein.
p.a. Pulmonary artery (cut short).
p.v. Pulmonary vein.

L.A. Left auricle.

L.V. Left ventricle.

d. ao. Dorsal aorta.
h.v. Hepatic veins.

c. Coeliac artery.

a. m. Anterior mesenteric artery.

b. r b. Adrenal body.

L.r.a. Left renal artery.
l.r.v. Left renal vein.

K. Kidney.

p.m. Posterior mesenteric artery.
spm. Spermatic artery (vein below).

c.il.a. Common iliac artery.

(b) Letters to left —

p._/". and a.f . Posterior and anterior
facial veins.

e. j. External jugular vein.
i j. Internal jugular vein.

R. Scl. Right subclavian artery.

S. V.C Superior vena cava.

R.A. Right auricle.

R.V. Right ventricle.

t.V.C. I nferior vena cava.

r. r.a. Right renal artery.
r r.v. Right renal vein.

s. r b Adrena' bodv.
spm. Spermatic artery and vein.

?'/./. Ilio*lumbar vein.
f.v. Femoral or external iliac vtin.
i.il.v. Interna! iliac veins.

THE RABBIT

555

start another contraction. The venous blood which
reaches the right auricle from the capillaries of the
body is driven by the auricular contraction into the
right ventricle and thence in turn through the pulmonary
artery to the lungs. Returning oxygenated to the left

Fig. 421. — Diagrams of the venous system

A, of a dogfish ; B, of an amphibian ; C, of a rabbit.

The primary system in grey ; that of the lateral and anterior abdominal veins in
black ; the inferior vena cava in white. The hepatic portal system is omitted.

1, Entry to heart ; 2, left superior vena cava or precaval or ductus Cuvieri ; 3, left
internal jugular or anterior cardinal ; 4, left external jugular or inferior jugular ;
5, left subscapular ; 6, left posterior cardinal ; 6', position of same in a newt;
in the frog the posterior cardinals are absent ; in the rabbit the portion shown by
dots is wanting ; 6", right azygos vein representing right posterior cardinal in a
mammal ; 6" left azygos vein ; 7, left deep lateral vein ; 7', pelvic ; 7", anterior
abdominal, representing both deep laterals fused ; 8, renal portal ; 9, caudal
(wanting in frog) ; 10, external iliac or femoral ; 11, internal iliac or hypogastric ;
12. inferior vena cava or postcaval ; 13, junction between azygos veins ; 14, left
subclavian.

auricle it is driven into the left ventricle, and thence
through the aorta to all parts of the body. There is thus
a double circulation, as in the frog, but the separation
of the ventricles and connection of the pulmonary artery
with one of them and the aorta with the other dispenses
with the elaborate apparatus of the truncus arteriosus.

556 MANUAL OF ELEMENTARY ZOOLOGY

The aortic arch bends over to the left and, as the dorsal
Arteries. aorta, passes backwards under the backbone
through the chest and abdomen till it becomes
the small caudal artery. A ligamentous band, known as
the ductus arteriosus connects the aortic arch with the
pulmonary artery, just before the bifurcation of the latter.
At one stage in development this band is represented by
an open tube (p. 635). In its course the aorta gives off
numerous arteries, of which the following are the most
important : (1) The innominate, arising from the top of
the aortic arch and dividing into the right subclavian and
right common carotid, the latter passing up the neck and
forking opposite the angle of the jaw into external and
internal branches, (2) the left common carotid, arising from
the aortic arch immediately beyond the innominate,1 (3)
the left subclavian, arising from the left side of the aortic
arch, (4) the c celiac, which arises from the dorsal aorta
shortly behind the diaphragm and divides into the hepatic
and the lienogastric, (5) the anterior mesenteric, shortly
behind the c celiac, (6) the renal arteries, (7) the genital
arteries, (8) the small posterior mesenteric, (9) the common
iliac arteries ; these last arise just before the hip girdle
and practically end the dorsal aorta, which after them is
diminished to the caudal artery.

Each superior vena cava is formed by the union of
veins. a. subclavian vein from the shoulder and fore-

limb, an external jugular from the surface of
the head, and an internal jugular from the brain. The
right superior vena cava receives also an azygos vein from
the walls of the chest. The external jugular is larger than
the internal and lies nearer the surface in the neck. The
inferior vena cava is a large median vessel which lies beside
the dorsal aorta. It receives the following veins : (1) The
internal iliacs or hypogastrics from the back of the thighs,
(2) the external iliacs from the inside of the thighs, (3) the
ilio-lumbars from the hinder part of the abdominal walls,
(4) the genital veins, (5) the renal veins, (6) the large hepatic
veins from the liver, through which organ it passes on its
way to the heart. Blood from the stomach, intestines, pan-
creas, and spleen is carried to the liver by the portal' vein,

1 Sometimes from the innominate itself.

THE RABBIT

557

but there is no renal portal system. The general course ot
the circulation of the blood in the rabbit is shown in the
table below.

Most of the lymphatic vessels are gathered up into a
thoracic duct which opens into the left subclavian vein at
its junction with the external jugular, but those of the right
side of the head and neck and right fore-limb communicate
with the venous system in the corresponding position on

Righr auricle
Right" ventricle

i

Lunas

1 .

Left" auricle

Left ventricle

Fig, 422. — A diagram of the circulation of the blood in the rabbit.
Thick lines indicate venous blood, narrow lines arterial blood.

the right side. (The arrangement of the main lymphatic
vessels of man, which is substantially the same as that of
the rabbit, is shown in Fig. 456.)

The blood of the rabbit differs from that of the frog and
dogfish in two important respects. (1) The red
Temperature corpuscles, instead of being oval in outline and
biconvex, with nuclei, are round and biconcave
and have no nuclei. (2) The temperature of the blood,
instead of rising and falling with that of the surrounding
air or water, is almost constant at about 38° C. This is ex-

55s MANUAL OF ELEMENTARY ZOOLOGY

!p,t ^ ^ 1 ' ,s;l- IJ18 that the rabbit is a warm-blooded animal
I he heat is produced, not in the blood, but in the solid
tissues, particularly m the glands and muscles, its appear-
ance accompanying the activity of the tissue. The circula-

t to. 423. The brain of a rabbit, seen from above with part of the
right cerebral hemisphere cut away.

■%.c

q'\ tni,en0r corp.us quadngeminum; a.ch.p., anterior choroid plexus- cb
.erebellum, cer.h. .cerebral hemisphere; crt., cortex; Ji ., flocculus * Vr /’
fronta1 lobe of cerebra. hemisphere; l.v., lateral ventricle; lat.L, later’al lobe
of cerebellum , medulla oblongata ; occ.L, occipital lobe of cerebral

hemisphere ; ol.b., olfactory bulb ; op.th ., optic thalamus; p.b pineal bodv ♦
P.c q., posterior corpus quadngeminum ; par.l., parietal lobf of cerebral hemi!
sphere, r. 3, roof of third ventricle; sp.c., spinal cord ; Sy.f. , Sylvian fissure-
tp.L, temporal lobe of cerebral hemisphere ; ver., vermis. J ^ ure '

(ion o f the blood, however, keeps the temperature of
different parts of the body nearly the same, the regula-
tion ot the temperature o i the body as a whole is brought
about by alteration in the production of heat and in the rate
at which it is lost. The principal means of increasing the

THE RABBIT

559

production ot heat is the activity of the muscles. Shiverin°-

circuktfoThr °rh ' i ' L°SS ,heat is Promoted by increased
heat t ?he he Se'n and uby sweatin£’ which absorbs

noted ?h^th P °? °f-the Sweat- We have already
ed that the power ot maintaining a steady temperature

Fig. 424 —The brain of a rabbit from below.

'm.oh., meduTlanoblOTgafltaTUi/V S/a’cto™ bu/b^ ol ]T' T'3™' hemisPhere ;
PW pituitary body° f^Ifpyr form° lobe ii P'V"

S&SKZfEr, 'jJSi.- ,emporal lote * cerebSh^rf STI.;

possess^ it^the ™ T' h (tHe ma“mals and birds) which
ra.ess n the great advantage that their activity is not

like that ot most creatures, closely dependent unon the

external temperature. It is one of the ways m whLh

higher animals have a greater mastorv than 1

over their surroundings. Y " l0Wer 0nes

56o MANUAL OF ELEMENTARY ZOOLOGY

The brain of the rabbit resembles that of the frog (p. 87)
in the main outlines of its structure, but there
System* are considerable differences in detail between
the two. The most conspicuous part is the
cerebrum, which consists of two very large cerebral hemi-
spheres divided by a deep cleft or median fissure , at the
bottom of which they are joined by a bridge known as
the corpus callosum , composed of nerve fibres, nearly all
of which run transversely. In the cerebrum the grey
matter has migrated from around the central cavity to

Fig. 425. — A semidiagrammatic, median, longitudinal section of

the brain of a rabbit.

aq., aquaeductus cerebri; c.c., corpus callosum; inf , infundibulum; m.c., midd!<
commissure, which connects the two optic thalami across the third ventricle ;
o.c., optic chiasma ; p.ch.p posterior choroid plexus; 3, 4, ventricles. Other
lettering as in Figs. 423, 424.

the surface of the pallium, where it forms a good cortex
(p. 89). It is almost smooth, but there can be seen on it
faint indications of some of the furrows or sulci which in
man are deep and numerous and divide the surface into
convolutions. Midway at the side of each hemisphere is a
shallow groove known as the lateral or Sylvian fissure ,
which separates a posterolateral temporal lobe from the
frontal and parietal lobes. On the under side a longi-
tudinal rhinal fissure marks off the frontal and temporal
lobes from a region median to them known as the rhinen-
cephalon , which consists of a pyriform lobe behind and the
olfactory lobe in front. The latter consists of the olfactory

THE RABBIT

561

tract and the olfactory bulb , which projects in front beyond
the frontal lobe.

In each hemisphere the pallium with its cortex extends over the
corpus striatum, where the grey matter remains internal. This

st

YCL$:

c&s.

rrrv

PlG. 426. — The solar plexus and neighbouring structures in a rabbit,

exposed by opening the abdomen and drawing the stomach to the
right.

m.nt.a., Anterior mesenteric artery; coel.a., coeliac artery; coel.g. , one of the
coeliac ganglia; d.ao dorsal aorta; i.v.c., inferior vena cava; l.r.a., left renal
artery (represented somewhat too large); Lr.v., left renal vein; lr., liver;
u^s., oesophagus ; rm., rectum ; sf>l.n., left splanchnic nerve; sr.b., suprarenal
body ; St., stomach ; sy.c. , sympathetic cord ; vag., left vagus.

disposition is due to a great expansion of an area (the neopallium ) of
the dorsal region of the pallium of lower vertebrates, which has
thrust apart the lateral and median regions. These now occupv
only small areas — the pyriform lobe on the ventrodateral aspect, and

36

562

MANUAL OF ELEMENTARY ZOOLOGY

the hippocampus , which has been tucked in on the median side and is
now mainly internal. With the development ot the neopallium is
connected the increased power of co-ordination possessed by the
highest vertebrates, and the great extent of this region in mammals is
accompanied by the ability to meet new situation- intelligently.

The thalamencephalon is overhung and hidden by the
cerebral hemispheres. Its thick sides form two large
thalami, and irom the hinder part of its thin roof the
pineal stalk passes backwards to end in the pineal body

pefil.

A.-mal

Fig. 427. —A diagram of the ear of a rabbit.

e./i.m., Externa' auditory meatus; Eu. , Eustachian tube: f.o.,
enestr.i ovalts , inc., incus; parts of the membranous

Itbyi inth, containing endolymph ; /ig., ligaments; mat.,

metbTaL^ ’ tympanic

between the hinder ends of the hemispheres. The
infundibulum is a funnel-like depression of the floor of
the thalamencephalon, which enters the pituitary body.
The latter, with the bottom of the infundibulum, Is usually
torn off in removing the brain from the skull, leaving a
longitudinal slit which leads into the third ventricle or
cavity of the thalamencephalon. A small, rounded,
median swelling immediately behind the infundibulum
is known as the corpus mammillare or corpus albicans.
The mid-brain is almost covered by the cerebral hemi-
spheres. Behind each of its optic iobes there is one of

THE RABBIT

56 3

auditory function, so that tour corpora quadrigemina or
colliculi are seen. The crura cerebri are more prominent
than in the frog. In the hind-brain, the cerebellum is
very large and much folded and consists of a median
lobe or vermis and two lateral lobes , each of which bears
on its outer side a small lobe known as the flocculus.
The lower side of the hind-brain is crossed in front by a
wide flat band ot transverse fibres, the pons J arolii , which
connects the two halves of the cerebellum. The medulla
oblongata, with the fourth ventricle in it, narrows back-
wards into the spinal cord. It is marked by a ventral
fissure bordered by two longitudinal bands ox pyramids.

There is no ventricle in the cerebellum, but small offsets of
the aquaeductus cerebri enter the corpora quadrigemina.
The third ventricle is deep, but very narrow, and is crossed
by a large middle commissure , which connects the thalami.
The lateral ventricles are wide, shallow, and curved.

The cranial nerves are twelve in number. The first ten
resemble those of the frog (p. 90) in origin and function,
but differ in details : thus the olfactory nerves arise from
the olfactory bulb as a number of fine threads which pass
at once through the openings of the cribriform plate
(p. 528). The seventh nerve, like that of the frog, has no
ophthalmic branch. The eleventh or accessory nerve arises
Irom the side ot the medulla and spinal cord by a number
of roots, the first of which is just behind the vagus and
the last at the level of the fifth spinal nerve. It supplies
certain muscles of the neck. The twelfth or hypoglossal
nerve also arises by several roots ; these are situated on
the ventral side of the medulla, outside the pyramid. Its
course resembles that of the hypoglossal (first spinal)
nerve of the frog. The spinal cord, its thirty or so nerves,
and the sympathetic system resemble essentially those of
the frog (pp. 86, 92). The sympathetic system has
two ganglia on each side in the neck, twelve pairs in the
thorax, and the same number in the abdomen. From the
hinder thoracic ganglion there starts a splanchnic nerve ,
which passes backwards into the abdomen and ends with
its fellow in a group of coeliac ganglia around the anterior
mesenteric artery. These ganglia, with numerous nerves
uniting and branching from them, constitute the solar

564 MANUAL OF ELEMENTARY ZOOLOGY

plexus (Fig. 426). A smaller plexus and ganglion of the
same kind lie around the origin of the posterior mesenteric
artery. A. number of important nerves belonging to all
these series are found in the neck (Fig. 414). Among
them are the following : (1) The hypoglossal, curving
forwards round the angle of the jaw, with a backward
branch, known as the raniUs descendens , which passes to
certain of the neck muscles ; (2) the vagus, running back-
wards outside the carotid artery and giving off a superior
laryngeal branch to the larynx, a depressor branch,1 which
arises near the superior laryngeal and passes backwards
beside the main vagus, and a recurrent or inferior laryngeal
branch, which loops forward round an artery and runs
beside the trachea to the muscles of the larynx ; behind
this the vagus passes backwards along the oesophagus ;
(3) the cervical sympathetic , lying beside the vagus and
depressor ; (4) the spinal nerves, of which the third gives a
great auricular branch to the ear and the fourth and fifth
give off branches which join to form the phrenic nerve to
the diaphragm. The vagus bears its vagus ganglion just
before it gives off the superior laryngeal nerve, and the
sympathetic bears near the ends of the neck its two cervical
ganglia.

The sense organs do not differ from those of the frog
(p. 98) enough to need special descriptions.
Besides the structures we have mentioned in
connection with the eye, there must be noticed
the lacrymal or tear glaatds , situated above the outer corner
of each eye, as well as Flarderian glands corresponding
to those of the frog. There are no Ftarderian glands
in man. The secretion of the eye glands flows over the
conjunctiva and passes into the nose by the nasal duct at
the inner angle of the eye. The structures of the outer and
middle ear have been mentioned (p. 530 ; Fig. 427). In the

Sense

Organs.

1 This nerve, which runs to the beginning of the aorta, receives its
name on account ot its function. It conveys from the aorta to the
brain afferent impulses, as a result of which, when the blood pressure
is too high, the central nervous system, acting through other nerves,
lowers the pressure of the blood by dilating the arterioles. In the
thorax, the main vagus sends branches to the heart. Through these
the beating of the heart is checked ; it is augmented through nerves
from the sympathetic system.

THE RABBIT

565

inner ear, there is present a large, spiral appendage of the
sacculus, known as the cochlea, which contains the endings
of those fibres of the auditory nerve which subserve the
sense of hearing. The ductus endolymphaticus ends in a
small saccus. In the nasal cavities (p. 539) the olfactory
epithelium is restricted to the upper part of the olfactory
chamber, the rest of the organ serving to warm and moisten
the air on its way to the lungs. Minute sense organs of
taste, known as taste-buds , are found in various parts
of the mouth, principally on certain elaborate papillae oi
several kinds on the tongue, and are supplied with fibres
which run in the glossopharyngeal and chorda tympani
nerves. The nature ol the sense of taste in the rabbit,
though perhaps not in lower animals, may be interred
from that of Man. In Man the only true “ tastes are
the perceptions of the qualities sweet, sour, bitter, salt,
and perhaps metallic and alkaline. Other so-called
tastes are aromas perceived by the organ ot smell, to
which traces of the substances that give rise to them are
conveyed by air from the mouth through the posterior
nares.

CHAPTER XXV

MAMMALIA

The rabbit is a member of a class of backboned animals
Mammalia * which includes man himself, and is on that

Prototheria’ account usually regarded as the “ highest ,r

amd^Eutheria grouP in the animal kingdom though it would
be hard to show that its members are more
highly organised than, for instance, the birds. They are

Fig. 42S. — The Duckinole ( Ornithorhynchm ). — From Thomson.

known as Mammals or Mammalia from their possessing
milk glands. They are warm-blooded, their skin bears
hairs, the heart consists of two auricles and two ventricles
and gives a single aortic arch, which curves to the left
side, there are two occipital condyles, the lower jaw
articulates with the squamosal bone, the sternum is seg-
mented, and the kidney is a metanephros. One little
group of mammals, found in Australia, Tasmania, and
New Guinea and known as Prototheria or Monotrematay
differs widely from the rest in that its members lay large,,

MAMMALIA

567

yolky eggs with shells, like those ol reptiles and birds.
Their temperature varies a good deal, and averages
little above 2s C., whereas that ol other mammals is
pretty constant at about 39° C.1 Their urinary, genital,
and anal openings discharge into a common cloaca,
and their shoulder girdle has well-developed precoracoids
and coracoids which meet the breastbone. I he Duckmole
( Ornithorhynchus ) is an example ol this group. It is a

FiG 420. — The shoulder girdle and breastbone of a duckmole.

cl. Clavicle ; cor ., coracoid ; g.c., glenoid cavity ; i.cl., interclavicle ,
p.c., precoracoid; r., ribs; sc., scapula, si., sternebr*.

small aquatic animal with webbed forefeet and a horny
bill, which lives in burrows in river banks in Australia.
Alf other mammals have minute eggs, which are not laid,
but undergo a great part of their development within the
body of the mother, from which they receive nourish-
ment. They have no cloaca or precoracoids, and the
coracoids are small projections ol the scapula. Among
them the Pouched Mammals, Metathena , or Marsupialia ,

1 The average temperature of man is about two degrees lower than
that of most mammals.

^68

MANUAL OF ELEMENTARY ZOOLOGY

stand apart from the rest. Their young are born in a very
immature state and carried by the mother for some time
in a pouch under the belly. They have a double vagina,
and the anus and urinogenital openings are surrounded
by a common sphincter. They are found principally in
Australia, where they form almost the whole of the wild
mammalian population, including the Kangaroos, Wombats,

Fig. 430. — Diagrams of the female organs of generation :

A, of a duckrnole (Monotremata) ; B, of a kangaroo (Marsupialia) ; C, of a rabbil
(Eutheria).

cl., Cloaca ; cl'., vestige of cloaca represented by common sphincter around opening
of vestibule and anus; F.t Fallopian tube (oviduct); o., internal opening of
oviduct; part., position in which a passage breaks through for the birth of the
young; ut., uterus; vag., vagina; vag.c., caecum of vaginae; vest., vestibule.

Note: in A no vagina, in B double vagina, in C single vagina. A further
development is seen in man (Fig. 455)> where there is a single uterus. Note also
the progressive disappearance of the eioaca from A to C.

etc, but the Opossums of America also belong to this
group. The remaining mammals, constituting most of the
class, are known as Eutheria.

The most aberrant of the Eutheria are the Cetacea or
Cetacea Whales and Dolphins — purely aquatic creatures
that live and breed in the water, to which they
are conspicuously adapted. Their bodies are fish-like in
shape and hairless, save for a few sensory hairs on the
head, but are protected by a thick layer of fat, known as

MAMMALIA

569

MANUAL OF ELEMENTARY ZOOLOGY

57

o

B

1"IG. 432.— Skulls of two whales.

A* f- d T-huC (Bau*.n? nyshcetus), a member of the Suborder

Mystacoceti,. which are without teeth and capture their food I>v
means of a fringe of whalebone. In life the lower end of the fringl
is tucked in between the tongue and the jaw. B, The Sperm Whale
(Physeter macrocephalus ), one of the Odontoceti. The^otted line
shows the outline of the. soft parts of the head. The teeth of the
upper jaw are vestigial. oe

- <4

MAMMALIA

sn

the blubber, under the skin. The fore-limbs are replaced
by paddles, in which, however, the bones of the arm and
hand can be made out, there are no hind limbs, the tail
bears a pair of fleshy flukes at the sides (not above and
below like those of a fish), and in some cases there is a
fleshy dorsal fin. The openings of the ears are relatively

minute and have no flaps, the eyes
are small, and the nostrils are placed
at the top of the head. This is in
connection with an arrangement by
which the end of the soft palate can
clasp the epiglottis and form a com-
plete tube from the nostrils to the
lungs, so that the animals can feed
and breathe at the same time. Thus
whales breathe air like all mammals,,
not water like fish. The so-called
spouting of whales is not the driving
out of water used in breathing, but
partly the getting rid of a little water
which has entered the nostrils and
mainly the condensation of steam in
the breath. Whales are carnivorous.
Some, like the Sperm Whale, have-
teeth, which are numerous, simple,
and all alike. Others are toothless
and provided with strainers of the
horny substance known as “whale-

Sc, Scapula with spin, bone, ” by which they obtain for food

the countless small creatures which
swarm in the surface waters of the sea.

Other aberrant groups of Mam-
malia, which can only be mentioned
here, are the Sea Cows or Sirenia *
aquatic animals which feed on water plants, and an assem-
blage of curious creatures, known as Edentata
Grou'sof because their teeth are defective or wanting,.
Eutheria. comprising the Sloths, Armadillos, and Ant-
eaters. The rest of the Eutheria fall into three
great series : the Hoofed Mammals or Ungulata , which are"
herbivorous ; the Hailed Mammals or Primates , which in

Fig. 433 — The left
fore- limb of Balct-
noptera , a whale-
bone whale. — From
Thomson.

, Scapula with spine
(S/>.) ; //., humerus ;

R., radius; U., ulna;
C., carpals embedded
in matrix ; Me., meta-
carpals ; Pk., phal-
anges.

572 MANUAL OF ELEMENTARY ZOOLOGY

most cases lead an arboreal life and feed upon fruit? eggs,
or other food which they find in trees ; and a less compact
assemblage of groups known as the Clawed Mammals or

4 3

Fin. 434- — The
bonesof the fore-
leg of a pig.—
From Thomson.

c.t Cuneiform ; A.,
humerus; ,
lunar (semilunar
or intermedium) ;
m., magnum; r.
radius; s., sca-
phoid ; t.y trape-
zoid ; u. , unciform;
3-5, digits.

Fig. 435.— The
bones of the
right fore - leg
of a calf, from
the outer side.
— From Thom-
son.

A,, End of humerus ;
me. 2.4, cannon
bone (fused third
and fourth meta-
carpals) ; me. 5,

fifth metacarpal ;

nodule ; r.,

radius; u., ole-
cranon process
of ulna.

Unguiculata , which are most often carnivorous in one way
or another. The broad distinctions between these groups
iie in the shape of their feet. The herbivorous ungulates
are comparatively defenceless and rely for their preservation

MAMMALIA

573

574

MANUAL OF ELEMENTARY ZOOLOGY

expressed by the

Irom the attacks ol carnivorous animals upon their turn of
speed, which is attained partly by their walking, not upon
the soles ol their feet, but upon the tips of their toes, so
that the power ol their limbs is concentrated. This is

statement that they are unguligrade .
Animals which, like dogs and cats,
walk upon the under surface of the toes
and never place the palm or instep
upon the ground are digitigrade. Those
which, like bears and man, walk upon
the whole sole of the foot are said to
be plantigrade. Those which, like the
rabbit, run upon the toes only, but
when at rest apply the whole sole to the
ground, are subplantigrade.

In Ungulata the metacarpal and meta
, 4 tarsal bones are lengthened,
so that what seem to be
“knees” are really the wrist and ankle
joints, high above the ground. The
first digit is wanting, and usually some
of the others are also missing. The
ends of those which remain are encased
in the broad horny coverings known as
hoofs. This does not apply to the
elephants, which have five toes with
short metacarpals and metatarsals.
Hoofs are very broad nails, which cover
the sides and part of the ends of the
toes. Tike other nails, they grow
from above downwards. The part

canTuml'^.i., cannon which covers the front and side of
bone (fused third and i he last phalanx (the “coffin bone”

of the horse) is formed by a thickened
ring of skin above it. known as the
cushion." That which covers the end of the
so-called “ sole ”) is formed by the whole
the skin it covers. In correspondence with
ungulates have broad grinding teeth, whose
surfaces are generally ridged, though in the omnivorous
pigs they are knobbed or buno don't. Ungulata fall into

Fig. 437*~~The bones
of the foot of an ox.
— From Thomson.

fourth metatarsals);
f>h., phalanges.

coronary
digit (the
surface of
their diet,

MAMMALIA

575

three main divisions: those of the even-toed forms or
Artiodactyla , comprising the pigs, cattle, antelopes, deer*

Fig. 438. — A side view of a sheep’s skull, with the roots of the back

teeth exposed.- — From Thomson.

t., Frontal ; n., nasal ; pm., premaxilla ; m., maxilla ; jugal ; sq., squamosal ;

lachrymal.

and camels ; the odd-toed forms or Perissodactyla , com-
prising horses, rhinoceroses, and tapirs: and the elephants
or Proboscidea.

Fig. 439. — The stomach of a sheep.-— From Leunis.

m., CEsopnagus , c. , rumen or paunch ; d. , reticulum or honeycomb- bag *
*■; psalterium or manyplies ; /, abomasum or reed; b., bes inning
of duodenum. ’ ®

The Artiodactyla are distinguished by the fact that the third and fourth
digits of each foot are equally developed, and the line which halves the
foot runs between them. Thus they have cloven hoofs. The premolars
and molars are usually different. The stomach is often complex and

576 MANUAL OF ELEMENTARY ZOOLOGY

the caecum is relatively small. The pigs and hippopotamuses, forming
the group Suina, are the least specialised of these animals. In corre*
spondence with their habit of dwelling in marshes and forests, where the
ground is soft and a broad tread is needed, they have four well-developed
digits on each foot, though the middle two alone touch the ground, and their
metacarpal and metatarsal bones are not fused into “cannon bones. ” The

dental formula of the pig is 3±_l> 4> 3. The canines are large, grow

3> U 4» 3

throughout life like the incisors of the rabbit, and in the male form
tusks ; the grinding teeth are knobbed, not ridged, and the stomach has
not the complicated form of that of animals that chew the cud. Cattle,
with deer, giraffes, antelopes, sheep, and camels, form the Ruminantia.
Here only the third and fourth digits are complete, and the fused

C

Fig. 440.— -A side view of a horse’s skull, roots of teeth exposed.

— From Thomson.

P.t Parietal; F., frontal; n, nasal ; //«., premaxilla; m.t maxilla; jugal ;
lacrymal ; sq , squamosal; paroccipital process; €., canine or

“ tush ” ; C., condyle.

me|;acarpals and metatarsals of these, digits form “cannon bones.”1
The fibula is represented only by a smalFmodule of bone attached -to
the distal end of the tibia. Usually there are no front teeth in the

upper jaw, the dental formula being — — -A: •?, The ridges of the grind-
• . 3> U 3> 3

sng teeth are crescentic and run fore and aft along the jaw. Such
teeth are called selenodont. The animals “chew the cud,” and
in connection with this habit have a complicated stomach, with
four compartments shown in Fig. 439. The food when it is first
swallowed passes into the rumen ox paunch at the left-hand end of the
organ, the walls of which are beset with'small processes or villi. Here
it is kept till the animal is ready to chew it, becoming meanwhile
«omewhat softened. It is then passed back into the mouth, chewed

1 Vestiges of the second and fifth digits are found in deer.

MAMMALIA

577

up. and mixed with saliva. When it is swallowed again it passes along
a muscular groove on the upper side of the second division of the stomach,
known as the reticulum or honeycomb-bag from the pattern on its
mucous membrane, into the psalterium or manyplies . The folded
walls of this chamber, covered with papillae, serve as a filter, through
which the food passes to the abomasum or reed, where the gastric juice
is secreted. Paired outgrowths of the frontal bones are common in
ruminants. In cattle, antelopes, sheep, and goats they are per*
manent and capped with hardened epidermis, the structures thus formed

Fig. 441.— - Diagrams of the structure of the horns of ungulata.

A., A growing antler ; B., the same fully grown ; C., the horn of an ox ; D.% the

horn of a rhinocerus.

b., Bone ; c., cutis; e., epidermis ; 4., born ; x., point at which bone is resorbed
in preparation for the shedding of the antler.

being known as horns. In deer they lose their skin and form purely
bony antlers , which are shed yearly. The horns of giraffes are short,
and, like antlers, are covered with skin, but this covering is permanent,
and the horns are not shed. (The horns of rhinoceroses are median
structures built up of epidermic fibres, and without a bony core.) The
camels and llamas have no horns, possess front teeth in both jaws, and
lack a psalterium in the stomach ; and, by a well known arrangement,
rows of little pockets, set like a honeycomb on the wall of the rumen,
serve to store water. The humps of camels contain a reserve of nutri-
ment in the form of fat. The Bactrian camel of Central Asia has two

37

578

MANUAL OF ELEMENTARY ZOOLOGY

humps, the Arabian camel has one.
the Arabian camel.

The dromedary is a swift race of

In the Perissodactyla the middle or third digit of each foot is larger
than the others and symmetrical in itself, and may be the only complete
■rflLA The premolars and molars are alike and have broad, transversely
ndged \lophodont) crowns. The stomach is simple, the csecum is laro-e

and there is no gall bladder. The rhinoceroses have three toes on each
loot, immensely thick and very sparsely hairy skin, and one or two
median horns, composed entirely of epidermic fibres adhering together
The tapirs have three toes on the hind foot and four on the fore foot'
hut the axis of each limb runs down the third digit, and not between the

Fig. 442. — -A transverse
section of an upper
molar tooth of a horse.
— From Theobald,
after Chauveau.

A, External cement ; B, ex-
ternal enamel ; C, dentine ;
D, internal enamel ; E , in-
ternal cement.

Fig. 443.— A vertical section of
an incisor tooth of a horse.
— From Theobald, after
Chauveau.

C, Cement ; E, enamel ; /, dentine.

ihird and fourth as in the Artiodactyla. The nose and upper lip are
drawn out into a short but mobile proboscis.

c f°Un? ln tw? wide]y separated localities, in Malaysia, and

South and Central America. Horses, asses, and zebras belong to the

E(lu^s' Here there is in each foot only one functional digit— the
third— with splints representing the metacarpals and metatarsals of the
second and fourth. The wrist of the horse is known as the “knee ”
die ankle as the “hock.” The metatarsals and metacarpals are the

« canf on J?oaes’ and three. phalanges of the single toe are the

pastern coronet or “ little pastern.” and “ coffin bone ”
respectively. The latter bears the hoof. The dental formula of the

h°rSe 1S 37i TsTs' The ridges on the grinding teeth are complicated,
*ome running along and some across the tooth. The enamel on the

mammalia

579

1 ig. 444. — A side view of the
lower part of a pony’s fore-
leg.— From Thomson.

h., Distal end of humerus ; u. ,
olecranon process of ulna ; r.
radius ; sc., scaphoid ; lunar ;
c., cuneiform ; os magnum ;
7m., unciform; p., pisiform;
me. 4, splint of fourth metacar-
pal ; me. 3, third metacarpal ;

i sesamoid; 1, 2, 3, phalanges
of third digit.

Fig. 445. — A side view of the
ankle and foot of a horse.
—From Thomson.

« . Astragalus; c., calcaneum;
n., navicular; e.c., external
cuneiform ; cub., cuboid ; mt. 3,
third metatarsal ; mt. 4, splint
of fourth metatarsal ; sesa-
moid ; ph. 1-3, phalanges of
third digit.

580

MANUAL OF ELEMENTARY ZOOLOGY

tips of the incisors is folded in, so as to form a pit. This gives rise to
a marking which alters as the teeth wear down, and enables the age of
the animal to be told.1

Fig. 446. The lower surface of a dog's skull.— From Thomson.

e.c.. Occipital condyle ; B.O., basioccipital ; T., tympanic bulla ; m.c .,
postglenoid process behind fossa for condyle of mandible; B.S., basi-
sphenoid ; P.S., base of presphenoid ; V., vomer ; M.i , second molar ;
M. 1, first molar; Pm. 1-4, premolars, the 4th the large carnassial ;
c., canine; /.1-3, incisors; Pmx ., premaxilla; mx ., maxilla;
palatine ; /., jugai ; A.S., alisphenoid ; Pi., pterygoid ; Sg,, squamosal
(the reference line points to the glenoid fossa).

1 The dates of appearance and replacement of the teeth are also
of use in judging the age of a horse. The incisors of the first pair are
tully cut at one month, all three pairs at a little over one year. The
first pair are replaced after two and a half years. The canines (tushes)

MAMMALIA

58 j

The Proboscidea or Elephants are generally classed with
the ungulates. They have all the five toes. The
trunk is a muscular extension of the nose, with
the nostrils at the end. The tusks are the two upper incisors
and are composed of solid dentine or ivory. There are
no canines, but the six grinding teeth are very large and
transversely ridged, and are developed one at a time, so that

VI

Fig. 447— The left side of a dog’s skull, from which the zygomatic

arch has been cut away.

as., Alisphenoid ; c., canine tooth ; e.a.m. ., external auditory meatus frontal ;
S-f 1 glenoid fossa; 1., incisor teeth; i.o.J, infraorbital foramen ; j\, jugal;
lac., lacrymal ; m., molar teeth ; mx. , maxilla ; na., nasal ; o.c., occipital con-
dyle (on exoccipital bone); os., orbitosphenoid ; pa., parietal \ pal., palatine; ;
pin., premolar teeth ; pmx., premaxilla ; pt., pterygoid ; s.o., supraoccipital
(with which is fused an interparietal) ; sg., squamosal ; sut., line of dots marking
the suture between the jugal and squamosal on the removed zygomatic arch ;
tympanic ; II.-V I., foramina for cranial nerves.

there is a succession of them, each being replaced as it
wears out. The testes do not descend into scrotal sacs.
The Hyracoidea, usually placed near the Elephants, are little
rabbit-like animals found in Palestine and Africa. Hyraxy
the “ coney ” of Scripture, is the best known example.

begin to show at four years, and are fully cut at five years, when all
the permanent incisors are level. At six years all the incisors show the
central enamel as a complete ring, and in the first pair the cement is
almost gone from within it. At eight years this pair shows the dentine
as a yellow line — the “ star.”

MANUAL OF ELEMENTARY ZOOLOGY

582

Fig. 448.— -Diagrams to show the difference in construction between
the wing of a bat and that of a bird. In the bat all the digits are
present, four of them are very long, and the wing surface is
provided by skin stretched from finger to finger and between the
arm and the flank. In the bird there are only three digits, these
are vestigial, and the wing surface is mainly provided by feathers
in rows parallel with the axis of the limb. — From Romanes.

MAMMALIA

583

Among the other groups of Eutheria are : the Rodentia , to
Uiiguicuiata, which belong rabbits, rats, mice, squirrels, etc.,
all characterised by two pairs of large incisors
adapted for gnawing and by the absence of canines ; the
Insectivora , to which belong moles, hedgehogs, and
shrews, with sharp-cusped teeth and a long snout ; the
Ckiroptera or bats, in which the fore-limb becomes a
wing by the lengthening of its digits and the formation of
a web of skin between them and the side of the body ; and

3 4

Fig. 449.— The Coccyx, or vestige of the caudal vertebrae
of man. — From Cunningham.

1, 2, Transverse processes; 3, for sacrum ; 4, cornu.

the Carnivora , to which belong dogs, cats, bears, and seals.
These latter are generally bold, intelligent animals. They
have claws, which in the cats are retractile. The canines
are strong and sharp, and some of the back teeth are
adapted by their narrow, blade-like crowns for cutting
flesh. 1 hese are the four upper premolar and the first

lower molar. The dental formula of the dog is 3 — i 4’ 2-

0 3? ij 3.

Dogs and cats are digitigrade, and, like most of ’the other
Carnivora, have five fingers and four toes. The clavicles
are rudimentary.

The last group of mammals which we shall mention is

584 MANUAL OF ELEMENTARY ZOOLOGY

the Primates , which include monkeys and man, together
Primates. with t^ie lemurs> which link the monkeys to
other mammals. The Primates are plantigrade,
and either their thumb or their great toe—usually both—
can be opposed to the other digits so as to grasp objects.
There are well-developed clavicles, and the upper arm
and thigh are free, as in the elephants, not enclosed in

m.t.s-

m.

Fig. 450.— The bones of the hard palate and upper permanent

teeth of man.

1

Anterior palatine foramen, or foramen incisivum ; c., canine tooth - i
incisor teeth ; molar teeth; m.p.s., suture between maxillary and palatine
bones, pff » posterior palatine foramen or foramen palatinum tnajus • fitn
premolar teeth ; s.n.p., spina nasalis posterior. * r •>

the skin of the trunk, as in most mammals. The orbits
are turned forwards, not, as is usual, to the sides, and,
except in the lemurs, are enclosed behind by a complete
bony wall The majority of these peculiarities are con-
nected with an arboreal habit. It should be noted that in
most respects the Primates, and man with them, are not
highly specialised animals. In their limbs, in their teeth,
in the possession ot clavicles, and in the alimentary canal

MAMMALIA

585

they present a type of organisation which is on the whole
below, rather than above, the average of specialisation in
the Mammalia.

|rTG 451.— The Dun-coloured Gibbon ( Hylobates entelloides).—
From Flower and Lydekker.

Man is related to a group of tailless, half-erect monkeys which includes
the gibbons, chimpanzee, gorilla, and orang-utan. From
man' these he differs far more strikingly by his mental attri-

butes than by his physical features, but the following points are of
interest. Man alone walks perfectly upright. His legs are longer than
those of the great apes, and the great toe is not opposable. He is less

Fig. 452 — A dorsal view of the pancreas and duodenum of man,
with the pancreatic duct exposed, showing its junction with the
bile duct, and the accessory pancreatic duct. — From Cunningham.

i, Pancreatic duct; 2, superior (anterior) mesenteric artery; 3, superior mesenteric
vein (branch of portal vein); 4, “head” of pancreas ; 5, branch of accessory
pancreatic duct; 6, bile duct ; 7, accessory pancreatic duct or duct of Santorini
communicating both with duodenum and with main pancreatic duct • 8 first
(superior) part of duodenum.

Fig. 453-— A man in the successive positions of walking.
— From Marey.

Note how first the left leg is straightened and tilts the
body forwards while the right leg swings to the front,
and then the right is straightened and the left swung.
See p. 476.

586 MANUAL OF ELEMENTARY ZOOLOGY

hairy. He has a better command over his voice. His brain is twice
the size of that of the gorilla, which in this respect approaches him

On<»«

MAMMALIA

587

Fig. 454.— -A hi

1. Mental foramen.

2 Body of the mandible.

3. Maxilla.

4. Ramus of mandible.

. Zygomatic arch.

. Styloid process.

7. External auditory
meatus.

3. Mastoid process.

9. Asterion.

10. Superior nuchal line

of occipital bone.

11. External occipital

protuberance.

an skull, seen from the
Cunningham.

12. Lambdoid suture.

13. Occipital bone.

14. Lambda.

15. Obelion placed be-

tween the two
parietal foramina

16. Parietal bone.

17. Lower temporal line.

18. Upper temporal line.

19. Squamous part of

temporal bone.

30. Bregma.

ai. Coronal suture,

si. Stephanion.

right side.— From

23. Frontal bone.

24. Pterion.

25. Temporal fossa.

26. Great wing of sphe-

noid. ;

27. Zygomatic bone.

28. Zygomatico - facias

foramen.

29. Lacrymal bone.

30. Nasal bone.

31 * 3 4 * * 7 * 9 10 11* Infru-orbkal foramen.
32. Piriform ' aperture
and anterior nasal
spine.

588

MANUAL OF ELEMENTARY ZOOLOGY

most nearly, and his cerebral convolutions are more complex than those
of the great apes. The cranial part of his skull is correspondingly en-
larged. When the face looks forwards the foramen magnum opens down-
wards, instead of more or less backwards, as in most other mammals.

2 12^

’ ’ ’ which is that of the great apes, but

gap between them

The dental formula is

2>i, 2, 3

ithe small size of the canines and the absence ot a

Tig. 45 5 a. — The lower
end of a vas (or duc-
tus) deferens of man,
with its seminal
vesicle. — -From Cun-
ningham.

■amp., “ Ampulla ” of vas
defeiens ; d.ej ductus
ejaculatorius ; v.d., vas
deferens; v.s., vesi-
cula seminalis.

I

4

Fig. 455B. — A diagram of
the human uterus in

longitudinal section. At
the lateral angles the
Fallopian tubes have been
cut away. — From Cun-
ningham.

i, Lateral angle of uterus ; 2,
cavity of body ; 3, cavity cf
cervix ; 4, vaginal cavity.

and the incisors are peculiar to man. The chin and the projection of
the nasal bones to support the nose are also human features.

The essential facts of human morphology may be summed up as
follows : Man is metazoan, triploblastic, chordate, vertebrate, penta-
dactyle, mammalian, eutherian, primate.1 Excepting only the limb-
skeletons, whose parts cannot be identified with those of animals
lower than the Amphibia (p. 474), the main outlines of each of his

1 See pp. 177, 275, 4 1 8, 421, 429, 56b. 568. 584.

MAMMALIA

589

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590

MANUAL OF ELEMENTARY ZOOLOGY

principal systems ot organs may be traced back, like those of other
mammals, to the fishes, as has been shown in the course of the fore-
going chapters.1 In comparison with the rabbit, the following features
of his anatomy may be mentioned, in addition to those which have
been stated in the last two paragraphs. Ilis tibia and fibula are
not fused ; the fore-arm is capable of pronation and supination
(Fig. 408); there is no centrale in the wrist; the bones of the
adult skull show considerable fusions, there being a single “ occipital
bone ” composed of basioccipital, exoccipitals, supraoccipital, and
interparietal, a sphenoid bone composed of basisphenoid, ali-
sphenoids, presphenoid, orbitosphenoids, and pterygoids, an

*lG- 457- — ' The caecum and neighbouring structures in man.— From

Cunningham.

1. Ileum ; 2, colon ; 3, caecum ; 4, appendix.

“ ethmoid bone ” composed of mesethmoid, ectethmoids, turbinals
and vomer, and a “ temporal bone ” composed of squamosal, tym-
panic, periotic, tympanohyal, and stvlohyal ; small, irregular
additional bones (Wormian bones) may be found in the course of the
sutures ol the root of the skull ; the pancreatic duct joins the bile
duct near the entiv of the latter into the duodenum ; the vermiform
appendix is a good deal smaller and the caecum very much smaller ;

1 In making such comparisons the student will find of use as
summaries ot the tacts, the following diagrams, etc. : Fi(rs 396

4T5> 4~ 1 5 4^4; 4'86, the second and third paragraphs on p iq and
Figs. 401, 405.

MAMMALIA

59i

the left superior vena cava (or innominate vein) joins that of the right
side before the latter enters the heart, forming thus a single superior
vena cava ; the external and internal iliac veins of each side join to
form a common iliac ; the fibres which form the depressor nerve of
the rabbit are included in the main vagus nerve • there is a single
pair of mammae, which are on the thorax; the uterus is a single
median structure ; the testes descend into an external scrotum ; a
seminal vesicle opens into the hinder end of each vas deferens ; and
the third eyelid is represented only by a slight, semilunar fold of
membrane, the plicce semilunaris .

CHAPTER XXVI

REPRODUCTION AND SEX

In the various animals which we have studied in the pre-
ceding chapters, we have come across a number
Reproduction: 0f examples of reproduction, occurring in
Asexual various ways and m different circumstances.

It will be well at this stage to consider the
process from a broad point of view. We have seen (pp. 13—
16) that the essence of every act of reproduction is the
origin, by fission from the body of an organism, of a repro-
ductive body, and the development of the latter into the
likeness of its parent, and that a process of reproduction is-
said to be sexual or asexual according as the reproductive
body does or does not undergo the process known as-
syngamy before it develops. Reproductive bodies which
differ thus in their behaviour have, with few exceptions,
a corresponding difference in their constitution. This
appears clearly on comparing the two kinds of reproduction
as they occur in Hydra. Asexual reproduction is relatively
a simple matter. It consists in the formation of a reproduc-
tive body known as a bud and its fission from the body and
development, although in Hydra the structural part of the
development is completed before fission takes place.1 In
the bud there are from the first both cells specialised
for services within the body— the ectoderm and endoderm
cells derived from the parent — and cells which are still
undifferentiated and will later give rise to germs, for the
bud will presently reproduce sexually. Thus the body
1 Except in the case of longitudinal and transverse fission.

59-

REPRODUCTION AND SEX

593

divides into two parts, but these are alike in composition,
each containing both potential germs and differentiated
body substance. Sexual reproduction is a more compli-
cated process. It involves the formation of reproductive
bodies which shall unite in syngamy before they develop.
Since in syngamy. two single nuclei unite, it can only be
carried out by single cells, and these, since they are
to reproduce the whole organism, must not be specialised
lor any function within the body (p. 150). Thus in
the formation of the reproductive bodies of sexual
1 eproduction we find a separation smglv ol cells which
are not specialised for service within the body from
those that are. A reproductive bodv which contains a
single nucleus and is not specialised for body services,
i> known as a g€v??z. Ova and spermatozoa are germs.
Reproduction by means of germs involves a longer
process of development,, which, unlike asexual repro-
duction, brings into being radically reconstructed in-
dividuals with a completely new' outfit of nuclei and
cytoplasm.

In the Metazoa, sexual reproduction bv means of ova
and spermatozoa always takes place. In some
ofe?he<J<iC*'0rt is also asexual reproduction, as wre have

Metazoa. seen in C hapter XII., but this is never the case
in the higher kinds, such as the frog. A number
ol Metazoa, howTever various insects, crustaceans, and
worms carry out a kind of reproduction wdiich, wdiile it
resembles the sexual process in being effected by germs, is
like asexual reproduction in not involving syngamy. This
is parthenogenesis or the development of unfertilised ova.
A e have met with it in the liver fluke and the green-fly.
In. certain kinds of water-fleas and of the minute wrater
animals known as rotifers fertilisation is not knowm
ever to take place. An important deduction that may
be drawn from the occurrence of parthenogenesis is that,
though normally two sets of chromosomes are concerned
in the development of an individual, one set by itself is
competent to direct this process.

With the reproductive processes ol Metazoa it is not
difficult to compare those of the Ciliata. These animals,
though they are not divided into cells, possess as wre have
38

594

MANUAL OF ELEMENTARY ZOOLOGY

seen, a body nucleus or meganucleus, and a germ
nucleus or micronucleus. The ordinary fission
®ePJJduction is asexual reproduction, each nucleus con-
frostoioa tributing to both offspring. The conjugation
of the Ciliata is an act of sexual reproduction,
consisting in the formation of germs, their union, and
a development of the zygote (p. 168). The difference
between the germ formation of the Ciliata and that of the
Metazoa is due to the absence of body-cells in the former,
where only the body nucleus perishes, the rest of the
parent body becoming a germ, as does the portion
which separates from it. Thus there arise two germs, of
which one has little if any cytoplasm and is a male
gamete, while the other, which keeps most if not all
of the cytoplasm of the parent, is a female gamete. After
true syngamy, there takes place a process comparable to
the development of a Metazoon. In the Sporozoa, also,
both sexual and asexual reproduction are found. The
trophozoites give rise to a number of germs which unite
in syngamy to form the sporonts. The latter give rise
asexually to the sporozoites, and from these, when they
have grown into trophozoites, there presently appear
sexual individuals again. There is not in the germ
formation of Sporozoa any such obvious freeing of germ-
from body-substance as is found in Metazoa and Ciliata,
but there are reasons for believing that in the discarding
of the residual protoplasm such a separation occurs. In
the Flagellata sexual reproduction is sometimes unknown
{ Trypanosoma , etc.). In Polytoma and Copromonas
syngamy occurs between ordinary individuals, and is
not part of an act of reproduction. There is no visible
distinction between body- and germ-substance at any
stage^ though it may be that matter which is com-
parable to the body-substance of the Metazoa is
destroyed before syngamy. In Amoeba proteus syngamy
is not known to occur. At the same time, if, as
may well be, the multiple fission of Amoeba proteus
resembles germ formation, in that there is left behind
in the residual protoplasm something that corresponds
to the body-substance of Metazoa, then this process
has the essential features of parthenogenesis, for in

REPRODUCTION AND SEX

595

it bodies of the nature of germs develop without
syngamy.

It will be gathered from what has just been said that,
in respect of their reproductive processes,
«fh8exuailam animals present a series, in some members of

^production, which, as in certain Amoeba and some
flagellates, sexual reproduction is not known
to occur, while in others it alternates more or less regularly
with asexual reproduction, and in yet others, as in the
frog, it replaces the latter entirely. We are faced with the
question : “ Why are there thus two modes of repro-

duction, and why does one come to replace the other as
we pass from lower to higher animals ? ” The facts may
be partly explained as follows. Asexual reproduction is
in two ways an easier process than sexual, (i) It is
simpler, since it often can make use of tissues and organs
already formed in the body of the parent, and always
reaches its end by a less roundabout route than the
development of an oosperm ; (2) it has not the handicap
of requiring the union of two gametes, in bringing which
about much energy is spent and loss sustained through
failure. It is not surprising, therefore, to find asexual
reproduction in animals whose tissues are not too specialised
for it. That sexual reproduction also takes place in most
of these animals can only be due to some necessity which
imposes it upon the organism. In the higher animals,
however, the cells which compose the tissues are so highly
specialised that the production of a new individual from
them is impossible, and thus asexual production is ex-
cluded and leaves the field to the sexual process or to
parthenogenesis. It remains to inquire what may be the
necessity which imposes the sexual process even upon
animals which have asexual reproduction.

Sexual reproduction has two primary characteristics.

0 One is, of course, syngamy; the other is the

from Germs, tormation oi ottsprmg from germs, that is, from
protoplasm which is not specialised for services
in the body. That this latter event occurs in the sexual
reproduction of Metazoa is evident (p. 593). In Protozoa
it is less so, since there it mainly affects the nucleus. In
Ciliata and Sporozoa, however, there is, as we have seen,

596 MANUAL OF ELEMENTARY ZOOLOGY

during gamete formation an observable discarding of
nuclear matter which has been used for the life of the
individual ( trophochromatin ), and there are indications
that in all cases such chromatin is separable from the
chromatin for the gametes {idio chromatin) and is destroyed
at gamete formation, so that the gamete becomes a germ
(p. 593) and the newly assembled nucleus of the zygote
has its own wav with the body. It must be in consequence
of a need for one or both of these events — -syngamy and
reproduction from germs — that sexual reproduction occurs.
Which, if either, is that whose necessity primarily imposes
it ? Now reproduction by means of germs is very im-
portant. In most if not in all animals it is necessary to
the continuance of the race, because protoplasm 1 specialised
for processes in the body becomes effete (pp. 150, 238) and
must be superseded by a fresh formation, and that is what
happens in reproduction from germs. It has indeed been
supposed that this is the goal of sexual reproduction, and
that syngamy plays a secondary part, probably as a
necessary stimulus to division (see below). But repro-
duction from germs does not come about onlv in the
sexual process. It is also achieved in parthenogenesis and
probably in various asexual processes in Protozoa
(endomixis, some spore formations, etc.). Thus it does
not, per se, necessitate sexual reproduction. It is syngamy
that is the primary function of that process : that the
reproduction should be performed by germs is incidental
to the syngamy, whose zygote must be free from material
that would hamper its development.

In itself syngamy is an act entirely distinct from repro-
duction, and has a precisely opposite effect,
since it lessens the number of individuals by
fusion whereas reproduction increases them by fission.
In a few Protozoa, as in Copromonas and sometimes in
Polytoma (pp. 152, 155), it takes place between adult
individuals (in which case it is known as ho loA gamy), and.
then it has no connection with reproduction, but usually
it takes place between germs specially produced for
multiplication, and thus it becomes a part of the repro-
ductive process, even though it halves the number of
1 In Protozoa perhaps, sometimes nucleoplasm only.

Syngamy.

REPRODUCTION AND SEX

597

adults that the germs could produce if they developed
asexually.1 So firmly, indeed, is it entrenched here that
most germs perish unless they perform it. It seems clear
from the very fact that it takes place in spite of the lessening
in numbers which it causes, and also from the elaborate
preparations which are often made for it, that it is in some
way useful, and not merely brought about by certain
circumstances without affecting the welfare of the being in
which it occurs. The problem is to discover what is the use
to the organism of this process which so often complicates
reproduction. With regard to this several theories have
been held.

(1) On account of the fact that the union of the germs
of higher animals is followed by the rapid nuclear division
which leads to the cell formation of development, it was
at one time held that syngamy is primarily a stimulus
to nuclear division , which is needed in certain cases. But
if that were so the nuclear division in Protozoa ought also
to be quickened by syngamv, whereas we find here that
syngamy usually takes place after a period of frequent
divisions and is often followed, as in Polytonia- , by a period
of rest in a cyst. Syngamy must meet some need other
than the stimulation of division. In the Metazoa it comes,
indeed, to act as a stimulus to division, but that is in-
cidental. The occurrence of syngamy between the germs
by which these animals reproduce is ensured by the germs
being unable to divide without it. Union accomplished,
however, the zygote undergo division to form the adult
body. That this happens is ensured by the fact that the
entry of the spermatozoon provides a centrosome which is
lacking in the ovum and imparts to the zygote a stimulus
to division, but this stimulus is not inherent in syngamy ;
it accompanies it in these cases. Nor is it necessary for
all ova. Those that are parthenogenitic can develop
without it.

(2) Another theory of the meaning of syngamy supposes

1 In the extreme case of Paramecium , where each parent forms only
two gametes, the halving of numbers by syngamy brings it about
that the zygotes are no more numerous than the parents, so that here
sexual reproduction produces new individuals without increasing the
total number, which grows only by asexual fission.

598 MANUAL OF ELEMENTARY ZOOLOGY

that by its means the vitality of the protoplasm is renewed .
We have already commented on the fact that the body
cells of a metazoon after a time become senile and die,
whereas an ovum and spermatozoon after their union
retain their vitality. It has been held that this is because
the zygote has been given a fresh lease of life by the
process of union of nuclei. Some observations upon
Paramecium and other ciliata were supposed to confirm
this view of the function of syngamy in the metazoa by
extending it to protozoa. But, as we have seen, this is a
mistake. The rejuvenating effects which often accompany
syngamy are due, not to the union of nuclei, but to
the previous freeing of the germs from the effete body
protoplasm.

(3) A third theory as to the meaning of this process
holds that its importance lies in the fact that it combines
the hereditary characters op different individuals. It is
well known that the individuals of a species of animal
differ from one another in respect of each of their charac-
ters. They are unlike in size, proportions, colour, in-
telligence, and so forth. It is also certain that many of
these differences are inherited ; the children of a tall man
will, for instance, be on the average taller than those of
a short man. In the sexually-produced offspring of
cross-fertilised animals these hereditary tendencies are
combined, so that the offspring are not exactly like either
parent. This happens in two ways. In many respects the
tendencies inherited from the two parents, when they
differ, do not counteract one another, but for a given
character of the animal in question, such as the size, or
shape, or colour, or texture, of any part of the body, one
or other of the parents is dominant , the legacy from the
other being latent or recessive , so that the offspring will
“ take after one parent. This dominance belongs to
one parent in the determining of some characters and
to the other in that of others, so that every syngamy
brings about a reshuffling of the characters, and the
young is not exactly like either parent. In respect of
other characters the tendencies inherited from the two
parents to appear to counteract one another in the
next generation, so that by syngamy these characters of

REPRODUCTION AND SEX

599

that generation become a mean of those of the parents.
Here also there will be a difference between the two
generations. Thus,1 it comes about that no two members
of a species are identical in their outfit of characters.
It is held that a change in the conditions of life — as of
food, temperature, enemies, and so forth — -is thereby
more likely to find individuals adapted — as by size, or
clothing, or strength — to cope with it and continue the
species, and this will clearly be of advantage. Now the
hereditary tendencies of organisms are, of course, con-
veyed by the reproductive bodies— in fertilisation, for
instance, by the ova and spermatozoa — and, as we have
seen (p. 138), this is mainly effected by the nuclei. Syn-
gamy, therefore, by the union of nuclei derived from,
different parents, maintains the unlikeness of individuals
which is held to be desirable for the species, and this,
according to the view we are discussing, is the function
by which the process is beneficial. This view is sup-
ported by the elaborate preparations which, in the
maturation of the gametes (p. 130 and Fig. 551), ensure
that, as regards the nucleus, the parents make equivalent
contributions to the zygotes which become the next
generation. The occurrence in some hermaphrodite
organisms (many plants, certain Protozoa, tapeworms,
etc.) of self-fertilisation is an obvious obstacle to this
theory, for the significance of syngamy in such cases
clearly cannot be that it benefits the species by the
combination of the characters of different parents. The
difficulty, however, is not necessarily insuperable, for, in
the first place, some self-fertilising organisms are occa-
sionally cross-fertilised, and, in the second, it may be that
self-fertilised organisms gain nothing by syngamy, but
are compelled to retain the practice by the fact that it
has become an inevitable step in the life-history.

In any case there can be no doubt that syngamy is a
Sex process of the highest importance to the

organism. If by nothing else, this is shown
by the fact that the whole mechanism of sex exists only
for its sake. Sex is primarily the differentiation of the
individuals of a species into tzvo kinds adapted to the

1 But see p. 703.

6oq

MANUAL OF ELEMENTARY ZOOLOGY

production of two kinds of gametes , though the differ-
entiation may extend to other functions, such as the part
played by the parents in bringing about the union of
the gametes or the nourishment and protection of the
young. It is an example of the physiological division of
labour between individuals. The basis upon which sex
is established is the unlikeness or “ dimorphism ” of the
gametes. In many protozoa, as, for instance, in Polytoma
and some other flagellates, so far as observation shows,
the gametes are identical (isogamous, p. 154). Nor is
there any difference between their parents. In some
species of Monocystis there is a dimorphism of the
gametes together with a restriction of the production of
each kind of them to certain individuals, which, however,
show no further difference. We have here the simplest
case of sex. The same thing is seen in some kinds of Hydra
and in medusae, where the sexes are alike in all respects
both of structure and of behaviour, save that one produces
only eggs and the other only sperm. In the frog the
parents differ, not only in respect of the gametes which
they form, but also in certain points of structure, such as
the generative ducts and the pad upon the hand of the
male, and conspicuously in their behaviour. One, the
female, plays in coition the same passive part which
the germs she forms play in syngamy. The other, the
male, plays the active part in coition and forms germs
which play the active part in syngamy. Throughout
the animal kingdom it is the case that, so far as there is a
difference between the sexes, the male is the active organ-
ism, the female relatively passive. It is the part of the
female to lay up the surplus material that she manufactures
in the formation of germs provided with the cytoplasm
which the zygote will require, and stocked with yolk to
serve as food in the early stages of development. Some-
times, as in mammals, she also nourishes the young formed
from the zygote. The part of the male is freely to expend
his substance in finding the female, while the germs that
he forms are poor in cytoplasm, but actively motile to
reach and enter the egg. In the highest animals his
activity is directed also to the preservation of the female,
for whom he fights and forages. It is in activity, not in

REPRODUCTION AND SEX

601

size or strength or beauty, that the true difference between
the sexes consists, though the female is perhaps generally
the larger and the male relatively the stronger of the two,
and the greater physiological vigour of the male is often
accompanied, as in the peacock and the stag, by the pos-
session of ornaments and weapons which are not found in
the other sex, and which play a part in courtship and in the
strife between males for the possession of females. Such
features as these are known as secondary sexual characters.
Sometimes, as in the newt (Fig. 353, A), they are developed
at the breeding season only. From the existence of sex
two benefits result. Firstly, a division ot labour of the
same kind that exists between the gametes becomes
possible between the individuals that iorm them, one
ensuring the union of the gametes, the other the nourish-
ment of the zygote. Secondly, it is ensured that cross-
fertilisation (p. 158) shall take place, since the germs
formed by one parent, being all of the same kind, are
unable to fertilise one another.

Dimorphism of the gametes is not necessarily accompanied with
the production of the two kinds upon distinct parents.
It happens fairly often that both sorts are formed by
p ro 1 ism. one individual, which is then, of course, called a her-
maphrodite. Paramecium presents such a case. Its gametes are
dimorphous. The migratory pronuclei represent male gametes ; the
stationary pronuclei represent female gametes. Each individual
produces a gamete of each kind. Some kinds ot Hydra also produce
two sorts of gametes in one individual, though in them the gametes
of one sort usually ripen before those of the other, so that the parent
has a kind of temporary sex. Hermaphrodites are sometimes self-
fertilised but usually, by such expedients as we have considered in
earthworms and snails, are cross-fertilised. In Vorticella , as we have
seen (p. 176), a curious imitation of sex is found. The parents are both
hermaphrodites, but of two kinds, one active and the other passive, like
parents which possess true sex.

The question of what causes an individual to be a male or a female
is an extraordinarily interesting and important one, but
The determma- tqe answer to it is still in many ways doubtful. The
difference between males and females is, in its essence,
of the same nature as other differences between members of a species,
and is known to be established in each generation according to the
same “ Mendelian ” laws as such differences (see pp. 699, 701)-
But the working of these laws is complicated in the case of sex by
quantitative phenomena, so that, e.g., a certain “ dose ” of the male
factor may be unable to overcome the female factor. One result of

602

MANUAL OF ELEMENTARY ZOOLOGY

this is the occasional appearance of “ intersexes ” — abnormal in-
dividuals intermediate between the sexes, or gradually passing from
one sex to the other, as a hen may with age come to resemble a cock
or even to breed as one. These individual are of quite a different
nature from hermaphrodites. The origin of hermaphroditism is at
present quite obscure.

It will be seen that syngamy is usually cross-fertilisation,
and that is caused to be so by various expedi-
fertiii'sation. ents, of which sex is one and mutual im-
pregnation by hermaphrodites is another.

This prevalence of cross-fertilisation might be explained
upon the theory that the importance of syngamy to the

organism lies in its increas-
ing variety by combining

different strains of the species.
On the other hand, it has
often been held to indicate

that for some physiological
reason a certain degree of
difference in the parentage
of the gamete is necessary
for the vigour of a zygote.
At first sight the latter of
these conclusions is supported
by the facts that in some
cases self-fertilisation of organ-
isms which are normally cross-
fertilised has been shown to give zygotes which either do
not develop or become degenerate adults, and again
that syngamy between gametes derived from near relatives,
or inbreeding , is often found to be harmful — a fact which is
well known to breeders, and is expressed in the marriage
laws of many nations. But there are animals known
in which self-fertilisation is the usual or the only kind of
syngamy that occurs, and others in which inbreeding has
no bad results whatever. The probable explanation of
these facts is that near relationship of gametes is not harm-
ful per se, and that the ill-effects which it usually has
are due to its concentrating any deleterious tendencies
which may exist in the race that is breeding in this
way.

Fig. 45S. — Sexual congress
in the snail, a cross-fer-
tilising hermaphrodite.

Fig. 459. — Secondary sexual differences in insects.

Above : the male (A) and female (B) of the Vapourer Moth, a species in which the
inactive habit of the female is exaggerated. Note her rudimentary wings,
heavy body, and small antennae, and the large, well-feathered antennae of the
male, with which he seeks her by scent.

Below : the male (with antler-like mandibles) and female of the Stag Beetle.

6oj

6o4 manual of elementary zoology

Stated in terms ot Mendelism (p. 698), what happens is that the
proportion of “ recessive ” characters, which are usually deleterious,
increases with each generation of inbreeding. However, in a con-
tinuously inbred stock the weaklings may be removed by a selective
death-rate and thus a strong race may result.

. This theory is confirmed by breeding experiments. The
virtue of cross-fertilisation thus appears to be that by
affecting heredity it confers variety and weakens harmful
factors a conclusion which is in agreement with the theory
that the function of syngamy is the modification of
heredity.

While cross-breeding within a species usually has
Mules. beneficial effects, gametes belonging to

different species, if the)7- will unite at all,
nearly always produce a mule — a hybrid which, though
it may in other respects be vigorous and hardy, cannot
reproduce its kind. This is probably because, though
their chromosomes can exist in the same nucleus, they do
not form homologous pairs, and thus cannot give rise to
gamete nuclei.

1

CHAPTER XXVII

EMBYROLOGY1

Hitherto we have been concerned almost wholly with the
anatomy and physiology of adult animals. We
Early1 stages must now give some attention to the process by
which the adult arises from the fertilised egg.
For this purpose we shall study first the development of
the lancelet, which is relatively simple and easy to follow
owing to the fact that the protoplasm of the ovum is not
hampered with a large amount of yolk. The egg is about
o-i mm. in diameter, and covered with a slight vitelline
membrane , which wall become more distinct when the
cytoplasm shrinks from it after fertilisation. Only the
first polar body is found on it when it is laid ; the second
is formed shortly after fertilisation. The ovum is one of
a kind in which there can already be distinguished portions
of the cytoplasm which are destined to form particular
regions of the future body, though in Amphioxus these
are not yet in position. Under the vitelline membrane is
a layer of cytoplasm which is finely granular but free from
yolk granules. Within this the cytoplasm contains yolk
granules. A large nucleus {germinal vesicle ) displaces
it from a good deal of one side of the ovum. This is the
animal pole ; the volky side is the vegetative pole. The
animal pole is usually spoken of as the upper side, but
actually when the embryo is formed this side will be
ventral and at the front end, while the vegetative side
will be dorsal and behind. When the egg is fertilised
certain changes occur. In preparation for the union of
the two sets of chromosomes, one from each gamete

1 In reading this chapter, the student should refer constantly to the

figures.

605

6o6

MANUAL OF ELEMENTARY ZOOLOGY

an ,k

nu^

nucleus, which takes place near the middle of the egg,
the nuclear membrane of the germinal vesicle breaks

down and a quantity of clear cyto-
plasm takes the place of the vesicle.
Meanwhile the granular cytoplasm
which has covered the ovum flows
to the vegetative pole and there
becomes a crescent around the future
posterior end. The clear cytoplasm
which now forms the animal side of
the egg will give rise to the ectoderm,
the yolky cytoplasm on the vegetative
side will give rise to the endoderm,
and the crescent of granular cyto-
plasm between the two at the hinder
end will give the mesoderm. The first
cleavage (Fig. 461, 2) is vertical and
forms two equal blastomeres (p. 137).
The second is also vertical, at right
angles to the first, the third nearly
equatorial, dividing each blastomere
into a rather smaller upper half and
chr. a rather larger lower half. The
' blastomeres do not meet in the
middle, so that at this stage they
form a ring. By repeated divisions,
vertical and horizontal, there arises a
sub-spherical (slightly pear-shaped)
structure, known as the blastula ,
whose cavity is the blastoccde. Its
wall consists of a single layer of cells,
which differ somewhat in different
regions. The ventral and anterior
part is composed of small cells
derived from the clear cytoplasm of
the ovum, the dorsal cells are larger
and more yolky, a crescent around
the posterior end is made up of smaller cells with granular
cytoplasm (Fig. 462, 2 and 3).

There now sets in a process known as gastrulation , by
which the blastula is converted into a two-layered structure,

gr * u rn

Fig. 460. — Ova of
Amphioxus .

A, Unfertilised; B, after
fertilisation.

an.p., Animal pole ; chr.,
chromosomes of the two
pronuclei, which have
met ; cl., clear cyto-
plasm ; gr., granular
cytoplasm ; nu., nucleus
(“ germinal vesicle ”) ;
p.b. 1, p.b.2, first and

second polar bodies ;
v.m., vitelline membrane;
y., yolky cytoplasm.

EMBRYOLOGY

607

the gaslrula (Fig. 462). This begins by a flattening of the
dorsal area of yolky cells. Next these are dimpled or
invaginated into the blastocoele,
forming a cup which, in spite ot
the difference in shape, may be
compared with the body of a
Hydra. The yolky cells — the
endoderm or hypoblast — form the
lining while the mouth or blasto-
pore, which is as yet very wide, has
a lip composed in front of small
cells of the anteroventral area, now
the ectoderm or epiblast , and at
the sides and behind ol the cells
of the crescent. The process con-
tinues by a rolling in over these
lips, so that in front ectoderm
cells and at the sides the cells
of the crescent become part of
the lining of the cup. Mean-
while the embryo is lengthening,
and the blastopore narrowing
owing to the growth backwards of
its anterior lip which, since it thus
comes to cover the dorsal side,
is known as the dorsal lip. The
gastrula when it has thus been
completed (Fig. 461, 1) is

elongate, with a small blasto-
pore at the hinder end of its flat
upper side. Its cavity is lined
below and at the sides by endo-
derm and has for its ceiling in
the middle a strip of cells which
rolled in over the dorsal lip and
will form the notochord, and on
either side of this a groove formed
from the crescent cells. These
cells are the mesoblast or the rudiment of the mesoderm.
The cavity of the gastrula is the ar chenterop . the blastocoele
has been obliterated during the invagination. Meanwhile

1. 2

Fig. 461. — Early stages of
the cleavage of the ovum
of Amphioxus .

r., Fertilised ovum ; cl., clear
cytoplasm which after cleavage
will be situated in the ectoderm
cells ; m., granular cytoplasm

which will be in the mesoderm
cells ; y., yolky cytoplasm

which will be in the endoderm
cells ; 2., first cleavage — stage
of two blastomeres ; 3., four

blastomeres ; 4., eight blasto-
meres ; 5., thirty-two blasto-

meres.

The figures are so posed that the
side which will in the adult be
dorsal is above and that which
will be anterior is to the left : the
horizontal axis of the adult is
indicated by the arrow in No. 1.
The animal pole of the ovum,
indicated by the (second) polar
body, is antero- ventral.

Fig. 462. — 'I he gastruiation of Amphioxus . — After Conklin, with

modification.

i, Sixty-four cell stage ; 2, completed blastula, from the left (and somewhat from
the ventral side, so that the mesoderm appears rather more dorsal than it is) ;

3, approximately the same stage, from the ventral side and a little from behind ;

4, right half of the same viewed from within ; 5, beginning of gastruiation •
6, later stage ; 7, late gastrula.

The figures are so posed that the side which will be anterior in the adult is to the
left, and (except in No. 2, which is a ventral view) the dorsal side is above. The
mesoderm cells are indicated by an arbitrary granulation.

608

EMBR YOLO GY

609

the cells of the ectoderm develop cilia, by means of which
the gastrula revolves within the vitelline membrane
and the cells of the flat dorsal side become more columnar
and form a distinct strip known as the neural or medullary
plate. The ectoderm at
the sides of this plate
now becomes detached
from it and grows over
it, enclosing a small
space (Fig. 464). This
process begins at the
hinder end, so that the
blastopore is covered
and opens into the
space in question (Fig.

463, 2 a). The sides

of the neural plate then
fold upwards and meet
above the space, so as
to form a tube which
will become the nerve
cord. Its hollow is the
neural canal , and the
blastopore, which is now
known as the neur-
enteric canal , leads from
it to the gut. Eventu-
ally the neurenteric
canal closes. An open-
ing, known as the
neuropore , long remains
at the front end of the
neural canal and puts
it into communication
with the animal’s olfac-
tory pit p. (417). While

these things are happening the ceiling of the archenteron
gives rise to the notochord and to the mesoderm.
The notochord is formed by a median longitudinal
strip (Fig. 464, 2) which becomes grooved and separated
from before backwards, its cells eventually rearranging

39

Fig. 463. — Longitudinal sections of
embryos of Amphioxus at stages
later than that represented by Fig.
462,. 7.

1, Vertical section of the completed gastrula
2 a, vertical and 2 b, horizontal sections of
an early post-gastrula stage in which only
the first mesoblastic somite has separated.
ach., Archenteron ; end., endoderm ; h.c., region
which will become the anterior median
pouch, or head cavity ; m-s., 1-3, meso-
blastic somites ; n.p., neural plate ; nch.,
notochord ; ne., neurenteric canal.

6io

MANUAL OF ELEMENTARY ZOOLOGY

themselves to form a rod (Fig. 467) and becoming
vacuolated. Its front end grows forwards to the end
of the snout. The hind end is for a long time con-
nected with the endoderm in front of the neurenteric
canal. The mesoderm arises as hollow outgrowths (Figs.
463, 464, 467). One of these is median and unpaired in
front. Behind it lies a pair of dorso-lateral grooves at the
sides of the notochord. The median pouch soon separates
from the gut, but the grooves, as fast as they close off in
front, are prolonged backwards. As growth progresses the
separated anterior part of the groove becomes segmented

Fig. 464. — Transverse sections of embryos of Amphioxus at stages
corresponding to Figs. 464, 1 and 2.- — After Hatschek.

acli., Archenteron ; mes.p., pouch which will become first mesoblastic somite ;
n.p., neural plate ; nch., rudiment of notochord.

into a series of pouches. These pouches are the mesoderm
segments or mesoblastic somites. They will presently
spread between ectoderm and endoderm and give rise to
the mesoderm of the adult. When the notochord and
mesoderm have separated, the dorsal edges of the endo-
derm close in under them to form a complete tube, the
enteron, which will become the alimentary canal of the
adult. The rudiments of all three layers' of a triploblastic
animal are now present. Ectoderm, endoderm, and
mesoderm are known as the germ layers , and though, as
we shall see, they arise in different ways in different cases,
they are present at an early stage in the embfyos of all

EMBRYOLOGY

611

Triploblastica. Before these
processes are complete,
hatching takes place by the
throwing off of the vitelline
membrane, and the embryo
becomes a larva (p. 32).
Until the formation of the
mouth the animal is some-
times called the “ free

Larva.

embryo.

Hatching takes place
about eight
hours after fer-
tilisation. About twenty-
four hours later the mouth
is formed as a small open-
ing, which rapidly enlarges,
on the left side of the fore-
part of the body. At the
same time, by perforation
from within outwards, the
first gill-slit is formed as a
median ventral opening
which shifts upwards to the
right side of the body. As
we shall see, this slit belongs
to the left side of the body
of the adult. The anus is
formed shortly after the
first gill-slit, much nearer
the hind end of the body
than it is in the adult.
Development henceforward
takes place more slowly, the
animal becoming adult in
about three months. We
will consider first the ex-
ternal features of its meta-
morphosis. More gill-slits
are formed in the mid-
ventral line, and each in

6 1 2 MANUAL OF ELEMENTARY ZOOLOGY

turn shifts on to the right side. They are primal y slits,
and each except the first acquires a tongue-bar. When
fourteen of these slits have been formed, another scries
appears above them on the right-hand side. These are
eight in number. Six of the first series ot slits disappear,
so that the number in the two series is the same. While
the formation of the second series of slits is taking place
both series shift downwards, the original series passing
over to the left side of the body, while the second series
remains on the right. At the same time the mouth also
shifts to its adult position in the middle line. The slits
at first open directly upon the surface of the body, but

Fig. 466. —Three larva stages of Amphioxus.— After
Lankester and Willey.

la A the metapleural folds are still separate; it. B they are united
behind ; in C they are united along their whole length.

«/., Atriopore; ciliated pit derived from the left anterior coelom ic
division of the ccelom ; g.e., gill-slits, left metapleural fold,

m.

mouth ; r./, right metapleural fold.

at an early stage, when there are as yet only six cleits of
the first series, two longitudinal ridges appear, one on
each side of the slits. In correspondence with the position
of the slits these ridges lie in front on the right side of the
body, but behind curve down to the ventral side, where
the new slits are forming. Ihese ridges are the meta-
pleural folds. From the inner face of each a secondary
ridge grows inwards to meet its fellow and enclose a space
below the body. This is the rudiment of the atrium. As
the ridges do not meet behind, there is lei t an opening
which becomes the atriopore. The closure ot the atrium
takes place from behind forwards as the folds shift down-
wards with the clefts, from the right side of the body to>

EMBRYOLOGY

613

their permanent position (Fig. 466). The atrium is at
first small, but enlarges so as to enclose the sides of the
pharynx. The endostyle appears at the beginning of the
larval period as a band of columnar ciliated cells on
the right side of the anterior end of the pharynx above
the first gill-cleft. It becomes folded as a V with the
apex directed backward. When the two rows of clefts
are established, the apex of this V grows back between
them, the two limbs fusing to form a single strip. As the
clefts move downwards the endostyle between them moves
also. A structure known as the club-shaped gland is
formed from the wall of the pharynx on the right side
above the gill-clefts. It disappears while the second series
of clefts is forming.

We must now consider the fate of the mesoderm rudi-
ments. The median outgrowth divides into
sonSteJ81'* right and left halves, of which the right becomes
a ccelomic cavity in the snout oi the adult,
while the left opens to the exterior and becomes Hatschek’s
pit in the wheel organ. Each ot the somites of the first pair
sends forward into the snout an outgrowth, which gives
rise to a cavity in the head of the adult, while its walls lorm
part of the mesoderm of the same region. The rest ot the.
somite gives rise to other spaces in the neighbourhood ot
the mouth and by backward outgrowths to spaces in the
metapleural folds. In the adult these latter spaces are
represented by lymph canals of doubtful origin. The walls
of the spaces give rise to mesodermal tissues around the
mouth and in the metapleural folds, and to the first myo-
mere. The remaining mesoblastic somites all behave
alike. They extend downwards on each side (big. 467)
till they meet below the gut. The outer or somatic . wall
of each lies against the ectoderm, together with which it
is known as the somatopleure ; the inner or splanchnic
wall lies against the endoderm and is known with it as
the splanchnopleure . The longitudinal septum or ventral
mesentery between the cavities of the two sides now
breaks down, so that they become continuous. Mean-
while there has formed in each of them a horizontal
septum which divides it into a dorsal half or epimere and
a ventral half, the hypomere or splanchnomere . The

614 MANUAL OF ELEMENTARY ZOOLOGY

cavity of the ventral portion is known as the splanchnoccele .
The septa between the splanchnocoeles break down so that
they form a continuous perivisceral coelom, which after-
wards becomes broken up in the pharyngeal region into a
series of tubes by the appearance of the gill-clefts (p. 406).
The cavities of the dorsal parts of the somites remain
separate and are known as my o codes. Their inner walls.

2 3

Fig. 467. — Transverse sections of embryos of A?nphioxus at successive
stages later than that represented by Fig. 462. — After Hatschek.

ent-y Gut or enteron ; m.pl., muscle plate ; m.s., mesoblastic somit ; myoc.,

myocoele ; n.c., nerve cord ; n.p., neural plate ; nch., notochord ; sop,, somato*
pleure ; splc., splanchnoccele ; spp., splanchnopleure.

against the notochord, become greatly thickened, each to
form a structure, known as a muscle plate , which gives
rise to a myomere in the adult, the walls between myocoele s
giving rise to connective-tissue septa between the myo-
meres, and the outer walls of the myocceles to the dermis.
From the inner wall of each epimere, below the muscle
plate, an outgrowth burrows its way between the muscle
plate and the notochord and forms from its wall the

EMBRYOLOGY

6 1 5

connective-tissue sheath of the
notochord and nerve cord. This
outgrowth is known as the
sclerotome , the main part, which
contains the muscle plate, being
known as the myotome. Lastly,
in the pharyngeal region the
myotome grows down in the
body wall, between the splanch-
noccele and the ectoderm, and at
its lower end forms an out-
growth, the gonotome , which
forms a gonad.

During the external and in-
ternal changes which
we have traced, the
larval Amphioxus
swims freely in the sea, usually
at a depth of a few fathoms
from the surface. As its meta-
morphosis reaches completion,
it sinks to the bottom and takes
up the burrowing habits of the
adult (see p. 418).

The ovum of the frog, its

Habits of the
Larva.

Fig. 468. — The development of the atrial
chamber in Amphioxus . — Alter Lan-
kester and Willey.

In I. the metapleural folds are seen sending a
slight projection inwards. In II. the two
projections have united and enclose a small
’ space ( A which is the rudiment of the
atrial chamber. In III. this space is
enlarging at the expense of the coelom.
A comparison of this figure with the
cross-section of the adult (Fig. 308) will
show the relation of coelom and atria!
chamber.

FR., C celomic space within dorsal fin; AL.,
gut ; S., coelomic space of metapleural fold ;
MP., metapleural fold ; SAT., projection
which forms floor of atrial chamber ; A O.,
aorta ; B.C. , coelom ; S.I.V., sub-intestinal
vein ; N., nerve cord; SH., sheath of noto-
chord; MY., muscle plate; C., cavity of
sclerotome; AT., atrial chamber. The
dotted line indicates the mesodermic wall
of the co lom.

//

MANUAL OF ELEMENTARY ZOOLOGY

616

fertilisation, and the beginning of its cleavage have been
described on pp. 136, 137. The first division of the
cleavage forms two similar cells, each containing, like

the ovum, an upper, black,
pigmented

segmentation, portion and a
lower, white,
yolky portion. The second
division is at right angles
to the first and forms four
similar blastomeres ; the
third division is horizontal
and separates lour small,
pigmented, upper blasto-
meres from four large,
yolky, lower blastomeres.
By succeeding divisions
sixteen and then thirty-
two blastomeres arise,
after which cleavage be-
comes irregular, the pig-
mented cells dividing
more rapidly than the
yolky. The final result
is the formation of a
blastula (Fig. 470), in
which the floor of the
blastoccele is composed of
large yolky cells and the
roof of small pigmented
cells. At the sides the
upper cells merge gradu-

Fig. 469. — Stages in the cleav-
age of the frog’s egg. Of
each stage two views are
given, one showing it from
the side, the other obliquely
from above. A-F show
successively stages with
two, four, eight, sixteen,
thirty-two, and numerous
blastomeres.

EMBR YOLOGY

617

ble..

ally into the lower. The large cells are the future
endoderm, the small cells will give rise to the ectoderm
and mesoderm, and both regions differ from the corre-
sponding parts of the blastula ot
Amphioxus in being more than one
cell deep, though the floor is much
thicker than the roof. From this
blastula a gastrula is formed, not by
invagination of the yolky cells into
the cup formed by the small cells,
which would be impossible on
account of the relative amount of
the two layers, but by an extension
of the small cells over the yolk cells.

Fig. 470.— A vertical
section of a frog’s
egg at the end of
cleavage.

blc., Blastocoele.

When this process begins, the two kinds
„ , , .. of cells each form half the
as ru a ion. outer surface of the blastula,

which floats with the black side uppermost.

The small, pigmented cells now start to

spread downwards, as a skin over the yolks cells, so that the
black area which the small cells form increases and the white area
where the yolk cells are exposed diminishes (Fig. 47 1 f This process
is known as epiboly. If it took place all over the surface ol the yolk

l.bl.

l.bl.

B

l.bl.

e

LbL

Fig. 471. — Stages in the gastrulation of the frog’s egg. The egg is
seen from the lower, white pole, which faces in the direction of
the future hind end of the animal.

l.bl., Lip of the blastopore ; y.p., yolk plug.

cells the result would be the formation of a close skin of small cells
over a solid mass of hypoblast without an enteron. But on one side
of the white surface, just below the boundary, on a greyish crescentic
area which arose there in the cytoplasm ot the ovum when the
spermatozoon entered on the opposite side, there appears at this
time a small, shallow, crescentic slit, convex towards the black area

Fig. 472. — Gastrulation in the trog.

A, B, and C, are sections in successive stages corresponding roughly to ' A, B and D
of Fig. 471. A' is a plan of the presumptive areas in stage A . C and C show the
rolling in of cells at the blastopore lip.

ar.y Archenteron roof formed by growth of the lip ; blc.y blastocoele ; end endoderm ,
ent., gut ; ep., epidermis ; blastopore lip ; mes., mesoderm ; n.p., presumptive
area of cells for neural plate ; nch., cells for notochord ; y.p yolk plug in blasto-
pore.

6x8

EMBR VO LOGY

619

(Fig. 471, A, l.b.l). Where the advancing black area reaches this its
further extension takes place in a different way, namely by the upper
side of the slit growing out and thus projecting as a lip-like fold over
the yolk cells on the other side. The result is that a narrow space is-
enclosed between the arched lip and the yolk cells (Fig. 472, B).
This process, which may be compared to the narrowing of the
blastopore of Amphioxus by growth of its lip (p. 607), is akin to
invagination rather than to epiboly. The side on which it happens is
the future dorsal side of the animal. The space enclosed is the
archenteron. The cells on the outer side of the lip are of course
continuous with the ectoderm. The cells of the lining of the lip
form the roof of the archenteron, its floor being formed by the large
yolk cells over which the lip is growing. As the lip extends, the rapid
advance of cells on its outer (upper or dorsal) side causes that side
to roll over at the edge, so that what was on the upper surface comes
under to form the archenteron roof. The arrows in Fig. 472, C' and
C", show the direction in which the cells move during this process.
The cell material thus transferred from the outside to the inside of
the roof of the archenteron during the formation of the gastrula is,
as we shall see, the material for the mesoderm and notochord.
When it was external it was not distinguishable from the epiblast,
but could be shown by staining methods to lie in strips ( presumptive
areas ) across the dorsal surface. As the small cells advance, these
areas converge upon the lip and turn in over it. The lip is the upper
border of the blastopore, which faces backwards : the rest of the
border is as yet indefinite and represented by the limit of the advancing
ectoderm all round the egg.

All this time the shape of the crescent is changing by its two ends
lengthening and curving towards one another till at last they meet h>
form a circle. By that time the edge of the ectoderm has reached this
circle all round its circumference, so that all the yolk is covered except
that within a circular area, the definite blastopore, bordered by a con-
tinuous lip and filled by a yolk plug consisting of yolk cells which have
not yet been covered. At this stage the embryo, which has hitherto
floated with the white pole downwards, begins to rotate so that
that pole moves upward around the aspect on which the dorsal
lip grew down. Towards this aspect will be directed the future
hinder end of the animal, and the remnant of the exposure of the
white cells — the yolk plug— comes, before it disappears (in the
way described below), to lie in the dorsal region of that end. By
the same rotation the region formed by the growth of. the dorsal
lip moves up into the position of the dorsal side of the animal. The
lip continues to grow over the yolk plug, thus narrowing the blasto-
pore. The narrowing, however, takes place not by equally rapid,
ingrowth of the lip all round, but by a faster growing together of
the sides of the circle in its hinder part. At last the plug is covered
and the blastopore is a mere slit. Then the middle part of this slit
closes completely, its sides coming together, but leaving at its ends
two small openings. Of these the upper remains open, the last
vestige of the blastopore, and becomes later the neurenteric canal
(p. 622), while the lower, though it closes, leaves in its place a pit ot

'620

MANUAL OF ELEMENTAL Y ZOOLOGY

•ectoderm the proctodaeum — in which the anus eventually breaks
through. . Where the sides ot the slit meet, between anus and
meurenteric canal, there remains a seam in the form of a groove —
'the primitive groove — under which lies a band oi cells— the primitive
streak in which ectoderm, endoderm and mesoderm meet and fuse.
During this process an inward movement of the yolk cells has
•obliterated the blastocoele and enlarged the enteron, which was at
first a mere slit, so that it becomes a spacious cavity, which com-
municates with the exterior by a slit between the dorsal semicircle
-ot the blastopore lip and the yolk plug. It is this change of site of

Fig- 473« — -The embryo of a frog shortly after the
completion of gastrulation, seen from the
right side arid somewhat from behind.

blp., Blastopore; n.f., neural folds.

the principal cavity in the embryo that, by shifting the centre of
..gravity, causes the rotation mentioned above.

At the end ot gastrulation the archenteron is a lar^e

'Forimtion of <ravit^ whose ver7 thick ventral wall has arisen
Mviesobiast. irom the yolk cells, while the thinner dorsal

wall has been formed bv the growth of the
blastopore lip. From the walls of 'the archenteron the
•endoderm proper (epithelium of midgut, p. 62s), noto-
•chord, and mesoderm arise as follows.

At each side the floor grows up within the roof as a
■sheet which eventually meets its fellow in the mid-dorsal
line and so shuts off the roof from the cavity. The com-
plete lining ot the gut which is thus formed from the yolkv
•cells is the endoderm proper. The layer, now outside it
o rmed by lip growth, becomes in the mid-dorsal line the
notochord and at the sides the mesoderm, which lies as
•a mantle around the endoderm. Because of the wav in
which the hp which formed it was completed, the meso-

EMBRYOLOGY

62 I!

derm mantle, which in front is dorsal only, behind
completely surrounds the endoderm. By growth of its-
fore-edge it spreads forwards and downwards between
ectoderm and endoderm, forming a layer all over the-
embryo, except in the mid-dorsal line, where the notochord
lies. It will be seen that the mesoderm first arises in the-
position in which the mesodermal grooves arise in
Amphioxus ; moreover, as in Amphioxus , the cells which
constitute it were from the first distinct from the endoderm
and reached their position later, by rolling in over the-
blastopore lip. On each side of the notochord the meso-

FiG. 474. — Sections of an embryo at about the stage of Fig. 473.

A , Vertical longitudinal ; B, transverse.

blc., Blastocoele ; end., volky cells of endoderm; ent., gut; mes., mesoderm ::
n.f., neural fold ; n.p., neural plate ; nch., notochord ; y.p., yolk plug.

derm is thicker than elsewhere, forming the segmental
plate. A split, the rudiment of the coelom, appears and
separates an outer or somatic layer from an inner or
splanchnic layer (Fig. 477 A). This split does not extend
far into the segmental plate, from which it soon disappears.
The segmental plates now divide into a series of blocks,,
the epimeres— usually, though incorrectly, called meso-
blastic somites ” — separating from the lateral plates, which
do not segment. The lateral plates correspond to the fused
hypomeres of Amphioxus (pp. 613, 614). The epimeres
form from before backwards, but the head region is not
segmented in the embryo of the frog, though it is so in the
embryo dogfish.

622

MANUAL OF ELEMENTAL A ZOOLOGY

Meanwhile external changes have been taking place.
External The emt>ry° at the end of gastrulation was still
Features of roughly spherical, the blastopore marking the
Embryo. future hind end. In front of this the future

■dorsal ectoderm flattens to form the pear-shaped neural
plate. The edges of this rise up owing to the incipient
formation of folds — the neural folds, which are continuous
in front, and behind join the side lips of the blastopore.
On either flank of the anterior end of the neural plate
appears a thickening which becomes divided by a furrow
into a gill plate and a sense plate which joins with its

fellow of the opposite
side. A ?ieural groove
appears along the middle
of the neural plate,
while the neural folds
grow taller, approaching
one another and deepen-
ing the neural groove,
bend over, and meet so
as to enclose a space,
the neural canal, uniting
first about the middle
of their length (Figs.

blp.-~

av..

Fig. 475* — An embryo of the frog at
a later stage.

■an., Proctodaeum (invagination which will form . h r . h * A ,oA A\ Q.i-nnr>

anus); blp., last vestige of blastopore gill 4 -7 5,47 7 N 4S0 A). MnCC

plate; « pi, neural fold; n.?., neural groove; they enclose the blaStO-
pnmi.ive groove ; S.A, »nse pla,.. ^ vestigg; {he ]atter

comes to lead from the
gut to the neural canal and gives rise to a neurenteric
canal, but this soon disappears. The neural canal separates
from the ectoderm above it, formed by the outer sides of
the neural folds, whose inner sides become the wall of
the neural canal. Before the folds have united in front,
the open canal between them is divided into three swellings,
the rudiments of the fore-, mid-, and hind-brains. It will
be seen that in the frog, as in the lancelet, the central
nervous system arises by the sinking in and folding of a
strip of the epidermis of the back. This process is found
m all Chordata, and is of the highest importance in the
drawing of comparisons between them and other animals.
During the formation of the central nervous system the

EMBR YOLO GY

62

e

s.

body has been elongating and other structures appearing.
Below the blastopore, in the area which it occupied before
its contraction, there appears, as we have seen, a pit
known as the proctodceum , and an opening piercing
through from this to the gut forms the anus. From anus
to blastopore runs a slight groove, the primitive groove.
Above it a knob grows out to form the tadpole’s tail.
Grooves appear on each gill plate marking out the visceral
arches, and upon the first
two branchial arches
branched external gills grow
out. Below the head, where
the sense plates joined, a
median pit of ectoderm
forms the stomodceum , which
will eventually break through
to the enteron and become
the mouth. Below the
stomodgeum is a horseshoe-
shaped sucker ; above, a
pit in each of the sense
plates gives rise to the ol-
factory organ. When these
changes are complete the
animal hatches. This
happens about a fortnight
after the eggs are laid.

In the external develop-

External ment of the tad“

Features of pole (Fig. Il)

Larva. the following

changes take place. A third

pair of external gills is formed, and the mouth opens

and is provided with a pair of horny jaws, which

are eventually lost. Meanwhile four gill-clefts open,

and the external gills wither, being replaced by new gills

on the walls of the clefts. The latter represent the first

to fourth branchial clefts of the dogfish, the external gills

standing on the first three branchial arches. Shortly

after the appearance of the clefts a fold of skin grows

back from each side of the head so as to cover them.

an

Fig. 476. — A still later embryo.

A, From behind and above; B , from in
front.

an.t Proctodaeum *, g.a gil! arches ;
s., sucker ; s.f ., sense plate.

624

MAX UAL OF ELEMENTARY ZOOLOGY

The folds are the opercula ; they meet ventrally, and
presently their hinder edges fuse with the body every wherer
except in one spot on the left side, where an opening is left

lr. mes.

Fig. 477.— Sections of an embryo frog at about the stage of Fig. 476.-

A, Transverse ; B, longitudinal.

<**., Anus cael., coelom ; ect., ectoderm or epiblast ; end., endoderm or hypoblast
f.br., fore-brain ; h.br., hind-brain ; ht., rudiment of heart ; int., intestine ,
l.p., lateral plate of mesoblast ; lr., rudiment of liver ; m.br., mid-brain ; m.s.,
mesoblastic somite ; mes., mesoblast ; mes'., mesoblast continuous with epiblast
of neural canal and hypoblast of notochord in region of primitive streak ; ne.c...
neurenteric canal (closing); nek., notochord; pk., pharynx; pit., rudiment of
pituitary body ; so.m., somatic mesoblast ; sp.c ., spinal cord ; sp.m., splanchnic
mesoblast; stm., stomod*um.

for the discharge of the water used in breathing. The-
sucker now begins to degenerate. Shortly afterwards,
rudiments of the hind-limbs appear at the base of the tail,
as a pair of small knobs, which increase rapidly and
become first jointed and then divided into toes. The-

EMBRYOLOGY

62 5

lore-limbs arise at the same time as the hind-limbs, but
as they are covered by the opercula they are not seen till
a later stage. About the end ot the second month the
lungs which have been forming come into use and the gills
start to degenerate, and a fortnight later the tadpole begins
to turn into a young frog. The outer layer of the skin
and the horny jaws are thrown off, the mouth enlarges
and changes its shape, the fore-limbs appear, that on the
left being pushed through the gill opening, that on the
right breaking through the operculum, the gill-clefts close,
and finally the tail shortens and is absorbed, and the
metamorphosis is complete.

Fig. 47S. — A frog embryo at the stage of hatching.

on., Proctodeum ; au.c., slight swelling over the rudiment of the ear ; e.e., externa?
gills on gill arches ; na., invagination to form nasal capsule ; o.c., slight swelling
over the rudiment of the eye; s., sucker; stm ., stomodaeum (invagination
which will form the mouth). •

We have traced the internal development of the embryo
Germ Layers. UP to the establishment of the three layers of
the body of a triploblastic animal. From the
embryonic ectoderm arise the epidermis, nervous system,
sense organs, and lining of the mouth and cloacal opening ;
from the embryonic endoderm arises the lining of the
greater part of the gut, the lungs, liver, pancreas, and
thyroid ; from the embryonic mesoderm arise the skeleton,
connective tissues, vascular system, muscles, excretory
organs, and generative organs. • The skeletal tissues and
unstriped muscle arise from a loose kind of mesoderm,
known as mesenchyme , formed mainly of cells which break
away from the compact layer around the coelom (chiefly by
a break-up ot the sclerotome), but also of cells which
40

626

MANUAL OF ELEMENTARY ZOOLOGY

migrate from the ectoderm and endoderm. The mass
around the coelom is known as mesothelium , and from it
arise all the remaining mesodermal tissues.

The origin of the central nervous system has already been
described. Owing to the shape of the surface
Nervous of the egg upon which the neural plate is
Sense-organs, formed, the fore brain is bent sharply down-
wards round the anterior end of the notochord
(Fig. 477 B). This is known as the cranial flexure. The

dorsal roots of the
nerves are formed as
growths from struc-
tures known as the
neural crests. These
are a pair of internal
ridges which project
from the sides of the
neural plate, near the
tops of the neural
folds, before the latter
have met. The parts
of the crests which do
not become nerve roots
are converted into
mesenchyme and
form, among other
things, the visceral
skeleton. The ventral
roots arise later, as
outgrowths from the
side of the central
nervous system, and

coel., Coelom; ect., ectoderm; gl., glomus, receiving ^ Spinal

a branch from one of the two suprabranchial COl'd become Connected
arteries which a little further back join to form
the dorsal aorta (cf. Fig. 485 a) ; int., intestine ;
lr., liver ; m.pl., muscle plate ; nch., notochord ;
nst., nephroccelomostome ; s.d., segmental
duct ; sop., somatopleure ; sp.c., spinal cord.

Fig. 479. — A diagram of a transverse
section of the frog embryo at the
hatching stage.

with the correspond-
ing dorsal roots. The
formation of the olfac-
tory organs has been
mentioned. The posterior nares arise from the olfactory
chambers as downgrowths which break through into the
mouth. The labyrinth of the ear is formed from the

EMBRYOLOGY

627

deeper layer of
the ectoderm as
an ingrowth
which forms a
vesicle, but doe§
not open to the
exterior. It
gradually takes
on the shape of
the labyrinth by
the formation of
septa which grow
into it and divide
it up. The eye
has a more com-
plicated origin.
The retina and
the pigmented
epithelium arise
from a pair of
outgrowths of the
fore-brain, known
as optic vesicles ,
which grow out
towards the sides
of the head soon
after the closure
of the neural
tube. Each takes
on the form of a
hollow bulb on
a hollow stalk.
The stalk gives
rise to the optic
nerve. The outer
half of the bulb
becomes thick-
ened and then
folded back into
the inner half, as
a hollow india-

Fig. 480. — Diagrams to illustrate the forma-
tion of the central nervous system of the frog.

A , The folding off of the neural canal (cf. Figs. 473-476) ;
ep., future epidermis ; n.c., neural crest, growing out
to form a dorsal root ; n.f., neural fold ; n.g., neural
groove ; n.p., neural plate.

B, transverse section of the fore-brain at the hatching
stage ;f.br., fore-brain ; Is., lens ; op.c., inner wall of optic
cup, which will form retina proper ; op.c'., outer wall,
which will form pigment layer; o.st., stalk of optic vesicle.

MANUAL OF ELEMENTARY ZOOLOGY

628

rubber ball may be folded when it has been punctured
(Fig. 480 B). The two-layered cup which thus arises is
known as the optic cup. The thick layer which lines it
is the retina, the thin layer on the side towards the stalk is
the pigment layer. From the deeper layer of the epiblast
there arises a thickening which projects into the mouth of
the cup, separates from the ectoderm, and becomes the lens,
after passing through a stage in which it is a hollow vesicle.

The alimentary canal arises from three rudiments :

the stomodaeum (p. 623) or fore-gut, which
canaf.n ary is of ectodermal origin and forms the mouth :

the mesenteron or mid-gut, which is endo-
dermal (p. 620) and forms the greater part of the
canal ; and the ectodermal proctodseum (p. 623) or hind-
gut, which forms the cloacal opening. The pituitary body
arises by an outgrowth from the roof of the mouth, meeting
the brain-floor (p. 64). The gill-slits are formed by out-
growths from the endodermal pharynx, which meet and
perforate the skin. The first of them, corresponding to
the spiracle of the dogfish, never opens, but forms the
tympanic cavity and Eustachian tube.1 Between, in front
of, and behind the clefts mesodermal thickenings con-
stitute the visceral arches, in which skeletal and vascular
structures corresponding to those of the dogfish arise.
The liver, pancreas, and lungs arise as ventral outgrowths
from the gut. The thyroid body starts as a median
longitudinal groove on the floor of the pharynx. This
gives rise to a solid mass of cells which separates from
the pharynx and divides into two. The intestine of the
tadpole, when the yolk in its ventral wall has been absorbed,
becomes for a time more coiled than that of the adult frog.,
probably in correspondence with the vegetable diet.

We have seen (p. 621) that the body cavity or coelom is
developed as a split in the mesoderm sheets.
TiSSesaSt'C Those cells of the splanchnic and somatic layers
which face towards this form the coelomic
epithelium. The greater part of the coelom becomes the
abdominal (pleuroperitoneal) cavity, surrounding the gut

1 Thi s is strictly true of many vertebrates (for instance, of birds and
mammals), but in the tadpole the first cleft disappears and where it
stood the Eustachian tube subsequently develops.

EMBR VO LOGY

629

Fi«

on all sides except in the mid-dorsal line, where the
mesentery is left. A forward ventral prolongation of the
■coelom becomes the pericardial cavity. The muscles of
the gut are formed from the splanchnic layer, the body
muscles from the myotonies, which, with sclerotomes
(p. 615), have arisen from the epimeres (“ mesoblastic
somites ”), though they are displaced in the adult. The
skeleton arises from mesenchyme, mainly from sclerotomes:
most of it is at first laid down in cartilage, which in places
becomes replaced by bone and in places is reinforced by
membrane bones (p. 40). The first rudiment of the cranium
has the form of a pair of curved
longitudinal bars, the trabecula p,
lying below the brain, and soon
continued back as a pair of para-
chordal plates at the sides of the
front end of the notochord, which
projects into the floor of the tad-
pole's skull as it does into that of
the dogfish. In most vertebrates,
the chick included, the para-
chordals are at first separate from
the trabeculae. The trabeculae fuse
with one another, leaving at first a
median space or “ fossa,” in which
lies the pituitary body, and with
the cartilaginous nasal and audi-
tory capsules ; and upgrowths from them form the sides
and eventually the roof of the cranium. The pituitary
■opening presently closes. The continuous palato-ptery go-
quadrate bar of cartilage, which forms a part of the
cartilage of the mandibular arch, is at first the only
.skeleton of the upper jaw. The hyoid apparatus of the
.adult is the remains of the skeleton ot the hyoid and the
basal parts of some branchial arches ot the tadpole. The
remainder of the visceral skeleton forms the cartilages of
the larynx.

The heart appears some time before hatching. It is at
„ . first a straight tube, which arises below the

pharynx. Subsequently the tube is thrown into
an S shape (see p. 442) and becomes divided by partitions

481. — A diagram of
the rudiment of the
skull in a tadpole.

iu.c., Auditory capsule ; i.c. ,
labial cartilage ; nch., noto-
chord ; /./., pituitary fossa ;
p.p.q., palato • p t e r y g c-
quadrate bar ; pch para-
chordal ; trb.y trabecula.

630

MANUAL OF ELEMENTARY ZOOLOGY

into its several chambers. The endothelium or pavement
epithelium which lines the heart arises by the rearrangement
of some scattered mesenchyme cells between the splanchnic
layer of mesoderm and the ventral endoderm of the gut, and
the muscular tissue is formed by a folding of the splanchnic
layer itself (Fig. 482). The space between the splanchno-

Fig 452. —A, B , and C, transverse sections through the ventral wait
of the throat of frog embryos of different ages, showing successive
stages in the development of the heart. — From Bourne.

tp., Epiblast ; hy., hypoblast ; tries., mesoblast ; eth ., endothelial lining of heart ;
ht., heart ; pc., pericardial cavity ; s., sucker ; so., somatic layer of mesoblast «
spl., splanchnic layer of mesoblast.

pleure and somatopleure in the region of the heart forms the
pericardial cavity. At this time it is continuous with the rest
of the coelom : later, communication between the pericardial
and abdominal cavities is abolished by the formation of the
great veins. The veins and arteries arise, by separation
of cells of the mesoderm, as irregular spaces which join up
to form blood vessels. The earliest vessels to appear are
the vitelline veins, one on each side of the yolk mass in

EA/BR YOLO GY

631

the splanchnic mesoderm. I hese join the sinus venosus
and carry to the heart the iood matter derived from the
yolk. The venous system is at first arranged on the same
plan as in the dogfish, with two ductus C uvieri and antenoi

V.

C.

Fig. .^.-Diagrams of the heart and chief arteries of a tadpole. -

From Bourne.

A The vessels of a tadpole at the stage when three external gills are present ; B,
' The ar'ang.ment when secondary gills are in use ; C, the adult arrangement.

Anterior commissural vessel; a.cb., anterior cerebral artery ’ “A’j (\ s?h and •
branchial arteries ; ao., dorsal aorta ; car., carotid artery , c.g , c; Series • ht ’
cu., cutaneous artery ; d.c., ductus caroticus ; ef., efferent branchial a[tenes *
heart; hy., efferent hyoidean artery ; 1., connecting vessel , f., lmgual artery ,
md„ efferent mandibular artery ; p.c., posterior comimssural vessel ; f UuPu mo-
cutaneous arch; pul., pulmonary artery; sys , systemic arch, tr., truncus
arteriosus ; v., ventricle ; I. -IV , branchial aortic arches.

a.c

and posterior cardinal veins. Subsequently the posterior
cardinal veins disappear and are replaced by the. inferior
vena cava, the ductus Cuvieri becoming the superior venae
cavae, and the anterior cardinals the internal jugulars
(Fig.’ 421). The vitelline veins are transformed into the

632

MANUAL OF ELEMENTARY ZOOLOGY

hepatic portal system and hepatic veins. The arterial
system of the tadpole closely resembles that of a fish. The
conus arteriosus leads into a long ventral aorta, from
the end ot which arise four vessels to the branchial
arches.1 From the gill capillaries there arises in each
arch an efferent vessel which discharges into a longi-
tudinal suprabranchial artery. The two suprabranchial
arteries, which are the earliest arteries to appear, are at

Fig. 484.— Diagrams showing how the arterial systems of adult verte-
biates are related to that of the embryo. The vessels are shown
from the ventral side.

A, Theoretically complete system of arches, not found in this form in any vertebrate,
adult or embryonic ; B, the system of the adult frog ; C, that of the adult bird ;
D, that of the adult mammal.

I -VI., Visceral arches; Br.i-Br.4, branchial arches; cod., cceliac ; d.ao., dorsal
aorta ; d.ar., ductus arteriosus ; e.car., external ( = ventral) carotid ; hd., hyoid
arch; i.car., internal (dorsal) carotid; Ing., lingual, or external (= ventral)
carotid; md., mandibular arch; pul., pulmonary; scl., subclavian; v.ao.,
ventral aorta ; 1 '.car., ventral carotid.

first entirely separate, but presently join behind to form
the dorsal aorta. In front they are continued as the
internal carotids. In the presence of a single efferent
vessel in each arch and of the two suprabranchial arteries,
the tadpole, while it differs from the dogfish, resembles
certain other fishes (p. 469). When the lungs are formed, a
vessel to supply each of them arises from the fourth efferent

1 There are traces of similar vessels in the hyoid and mandibular
arches.

EMBRYOLOGY

633

branchial vessel of the same side. Belore the gills are
lost, direct communication is established between the
afferent and efferent vessels, so that when the gill capillaries
disappear blood can pass direct from ventral to dorsal
aorta through four continuous aortic arches. After the
loss of the gill capillaries certain parts of the four arches
disappear, while other parts persist and become the great
arteries of the adult. The first branchial arch becomes

Fig. 485. — Diagrams of the development of the excretory system of

the frog.— From Bourne.

A, The system of a tadpole about 12 mm. long, showing the pronephros and origin
of the mesonephric tubules ; B, the system at the end of metamorphosis. The
broken line represents approximately the position of the strip of peritoneal
epithelium which gives rise to the oviduct.

cl., Cloaca ; d.ao., dorsal aorta ; f.b., fat body ; gl., glomus ; g.r., genital ridge ;
m.s., mesonephros; ms.t., mesonephric tubules; od., oviduct; ovf., position of
oviducal opening ; pn.f., pronephric funnels ; pnp., pronephros ; sg., segmental
duct (the line points to the part which becomes the Wolffian duct).

The pronephros is shown diagrammatically in transverse section in Fig. 626.

the carotid. The portion of the suprabranchial artery
which connected it with the arch behind it is usually
obliterated, but sometimes there remains a trace of it
known as the ductus caroticus. The second branchial arch
becomes the systemic arch. The third branchial arch
disappears altogether. The fourth branchial arch be-
comes the pulmo- cutaneous. It loses its connection with
the aorta save for a vestige in the form of a ligament in
most adults, but sometimes, as always in the newt, an

634

MANUAL OF ELEMENTARY ZOOLOGY

•b

>

Q.

O

EMBRYOLOGY

635

open connection persists, and is known as the ductus
arteriosus } We have seen (p. 556) that it is present also
during the development of the rabbit, where a vestige
remains in the adult.

The rudiment of the excretory system arises before
hatching, as a longitudinal thickening of the
Excretory and somatic mesoderm on each side at the front
organs!'Ve end of the coelom, immediately below the
myotonies* The mesoderm here is the inter-
mediate cell mass. In many animals it is composed of
nephrotomes , one tor each myotome. Its tront part gives
rise to the pronephros (p. 439), which consists ot three
twisted tubules each opening into the coelom ( nephroccele )
of the intermediate cell mass. Opposite the openings
(nephroccelomostomes) a sacculated outgrowth ot the
splanchnic layer appears. It is known as the glomus 2 and
becomes filled with blood from the systemic arch. The
outer part of the thickening for the kidney becomes a
longitudinal tube, the segmental or archinephric duct, into
which the pronephric tubules open at their outer ends.
This duct grows backwards and at the time ot hatching
opens into the cloaca. Later the adult trog s kidney
(“ mesonephros,” see p. 636) arises as a series ot paired
masses of cells in the kidney rudiment along the inner side
of the segmental duct, behind the pronephros. The part of

1 The term ductus Botalli is often applied both to the ductus
caroticus and to the ductus arteriosus.

2 This is exceptional. Usually each tubule has its own glomerulus,
enclosed in a nephroccele so as to form a Malpighian capsule (p. 78)-

FlG. 486. — A diagram of the kidney tubules and ducts of vertebrata.

A., Condition in larva of fishes and amphibians (pronephric tubules shown with
separate glomeruli), op. rud., Developing rudiment of opisthonephros ; pn.,
pronephros ; seg.d., segmental duct. B., Condition in adult fishes and amphibians.
G., gonad ; op'., part of the opisthonephros which in the male is connected with
the testis ; op"., part of the opisthonephros which is purely urinary in both sexes ;
nr.d., urinary duct of male dogfish and newt ; v.e., vasa efferentia ; W.d.,

Wolffian duct. C., Condition in the embryos of reptiles, birds, and mammals,
after the degeneration of the pronephros. G., gonad ; mes'., part of the meso-
nephros which in the male becomes connected with the testis ; mes"., rest of
mesonephros ; met. rud., rudiment of metanephros j ur., rudiment of ureter,
W.d., Wolffian duct. D., Condition in the adult of reptiles, birds, and mammals.
ep., in male, epididymis, in female a vestige known as epoophoron ; G., gonad ;
mes.v., vestige of hinder part of mesonephros, known in male as paradidytnis, in
female as paroophoron ; met., metanephros (possibly formed by the branching
of one tubule only) : ur., ureter ; v.d., vas deferens (in male only).

636

MANUAL OF ELEMENTARY ZOOLOGY

the segmental duct in this region is now called the Wolffian
•duct. Each of the cell masses develops into one of the
kidney tubules, having at one end an opening to the
Wolffian duct, and at the other a Malpighian capsule
(secondary nephroccele) with a glomerulus, and beyond the
capsule a “ peritoneal funnel ” which opens to the main
coelom. In later stages this funnel loses its connection with
the tubule and acquires instead a communication with a
vein which it retains in the adult.1 Outgrowths from
certain Malpighian capsules to the testis form the vasa
efferentia.

The frog, as we have seen, lacks the hind-kidney or metanephros
which reptiles, birds, and mammals possess, and which in them
forms alone the kidney of the adult (see pp. 439, 547, 548). In
the latter classes of animals the set of kidney tubules which constitute
the metanephros develop in a portion of the kidney rudiment which
lies behind the mesonephros, form no connection with the testis,
•do not at any time open by a funnel to the coelom, and discharge,
not by the Wolffian duct, but by a special duct, the ureter. This
duct arises by growing forward from the hinder end of the Wolffian
duct, branches to join the metanephric tubules, and eventually loses
its hindward connection with the Wolffian duct, and comes to open
independently into the cloaca (or bladder). Strictly speaking, what
we have called the mesonephros of the frog is an opisthonephros
(p. 440, footnote), corresponding to both the meso- and meta-
nephros of animals which possess the latter. All the tubules of
the frog’s kidney, however, have the connections of those of a
mesonephros.

Just before metamorphosis the pronephros and the front
part of the segmental duct degenerate. The oviduct arises
as a structure called the Mullerian duct , which is present
in the late tadpole in both sexes, but degenerates in the
male, leaving only a minute vestige. It is formed as a
longitudinal tract of the peritoneal epithelium outside the
kidneys, which becomes converted into a canal, the
front part by being grooved and then closing in, the
hinder part by hollowing out. Part of the groove does
not close, but remains as the internal opening of the
■oviduct. The gonads are formed as thickenings of the

1 In this condition of the peritoneal funnels, frogs are unique. It
is probably, like the lymph-hearts, an expedient for facilitating the
return to the blood vessels of lymph, which in the frog is copious and
less confined in vessels than that of higher vertebrata.

EMBRYOLOGY

637

coelomic epithelium, one on either side of the mesentery,,
on the dorsal wall of the peritoneal cavity. No distinction
between the sexes can be seen until the metamorphosis-
takes place.

The difference between the mode of cleavage of the
ovum of the lancelet and that of the ovum of
Segmentation, the frog is due to the presence in the latter
of a considerable quantity of yolk or food
material stored to provide for the nourishment of the em-
bryo during the earlv stages
of development. This yolk,
lying on one side of the
egg, hampers the relatively
scanty protoplasm there,
which therefore divides
more slowly, and, as we
have seen, forms fewer and
therefore larger cells than
the other side. In the dog-
fish and in birds there is no
food-procuring, larval stage
of development, but the
embryo is nourished within
the egg until it has sub-
stantially the features of the
adult. Accordingly the yolk
is still more plentiful, with
the result that the portion
of the egg in which it is
stored never divides at all,
but remains as an inert mass until it is surrounded by the-
growth of the small protoplasmic region or germinal disc ,
which containing the nucleus, lies originally at one pole
(Fig. 487), and segments to form the cells of the embryo.
The cleavage of the ovum of the lancelet is complete or
holoblastic and almost equal ; that of the ovum of the-
frog is holoblastic and unequal ; that of the dogfish and
birds is incomplete or meroblastic.

The egg of a bird — for instance, that of the common
fowl (Fig. 487) — consists of an immense ovum, the-
so-called u yolk," with certain coverings. The ovunr

non of the egg of a bird. —
From Thomson.

a.c., Air chamber ; ch., twisted cords-
in the white known as “ chalazae ” ;
g.v., “ germinal disc,” a small patch
of protoplasm, comparatively free
from yolk, in which lies the-
“ germinal vesicle ” or nucleus ;
y., yolk, in alternate layers of
yellow' and white substance. The
yolk is surrounded by the “ white
of egg.” Note the twm membranes
underlying the shell and separating
to enclose the air chamber.

638 MANUAL OF ELEMENTARY ZOOLOGY

owes its size to being swollen, as we have just, seei
by a great quantity of nutritive materia

oHhe°chickt: tbe proper, which pushes the bulk c

Early stages, the protoplasm to one side as the germing
disc. It is covered, first by its closely-fittin
vitelline membrane ; then by the “ white,” which i
a solution of proteins and salts, representing the jell
around the frog’s ovum • then, by a double mem
brane, whose two layers, when the egg has been lai<
for some time, part at the broad end and so enclose at
air-space; and finally by a porous, chalky shell. Th.
white provides, an additional store of nutriment, but it
principal function is to serve as a store of water, withou

Fig. 488.— Views of the young blastoderm of the chick in three

successive stages of cleavage.

A, Two-cell stage. B, Four-cell stage. C, Eight-cell stage.

which the embryo could not develop on dry land Thu(
oirds and reptiles are freed from that dependence upon £
watery medium during their early stages which exists foi

eavage (Fig. 488) begins with the formatioi:
ac rass the germinal disc of a furrow which does not reach

furrow dndCtbedge' ThiS iS SO°n CrOSSed bF anothei
furrow, and then more appear till the disc is divided intc

a mosaic of small irregular segments (Fig. 49o, 1). Sections

f th^ dlf° sh°^ that at the same time a horizontal cleft F
forming by which the segments become separated from the
underlying yolk (Fig. 489, B). By a further series of
lonzontal clefts the disc becomes two or three cells deep (C)
n that way, shortly before the laying of the egg a cup of

IhekoTis a the bla\toderm is fofmed. Between it and
• 'Olk is a space known as the subgerminal cavity

EMBR YOLO GY

639

The cells at the surface now become fitted regularly
together and form an upper layer , one cell deep, separated
by a chink, the blastocoele , from the rest, which are rounded,
and torm an irregular stratum, the lower layer (Fig. 491, D).
The upper layer will give rise to the ectoderm (epiblast)
and to the primitive streak ; the lower layer is the endo-
derm (hypoblast) and will give rise to the lining epithelium
ol the gut and of the various organs which will form as
outgrowths from that (p. 625). The subgerminal cavity will

Fig. 489. — Sections through the young blastoderm of the chick in
three successive stages, of which the first (A) is that of Fig. 488, c,
and the second (B) a little later than that of Fig. 490, 1. Slisiitly
diagrammatic.

hi-, blastomeres ; s.g.c., subgerminal cavity ; v.m., vitelline membrane ; y., volk.

become the hollow of the gut. Around the edge the lower
layer cells are more closely packed, and rest on the yolk,
which in this region contains nuclei derived from them
and is known as the germinal wall (Fig. 490, 2, g.w.).
At this stage the egg is laid, and the low temperature
outside the body of its mother brings its development to a
standstill. When, however, the mother begins to sit, or
the egg is put into an incubator, so that the temperature
rises, changes take place rapidly. The blastoderm spreads

640 MANUAL OF ELEMENTARY ZOOLOGY

v

EMBRYOLOGY

641

over the yolk, chiefly owing to the addition to its margin
of cells formed around nuclei of the germinal wall, which
recedes, by growing on the outside while it forms blasto-
derm on the inside. In a surface view of the blastoderm
there is now a dark, translucent area pellucida (light in
Fig. 490) over the subgerminal cavity, and a whiter area
opaca where the blastoderm rests on the yolk. It is in
the area pellucida that the body of the chick will be

?/>■

Y* S v * % C » . ••• - -V /• l 'X:M • » '

' 1 S * I ^ -‘.A ■

blY ' hyp -

Fig. 49 ^ Sections through the blastoderm of the chick in two stages
(D, E) which succeed that of Fig. 4S9, C, slightly diagrammatic.

blc., blastocoele ; ep., ectoderm ; hyp., endoderm ; l.l., lower layer cells ; mch., cells

which will join the mesenchyme.

Fig. 490. — Stages in the development of a chick. — After Marshall.

1. Segmentation ; superficial view of blastoderm. 2. Longitudinal vertical section of
later blastoderm ; blc., Blastocoele ; ep., ectoderm ; g.w., germinal wall ; lx.r
lower layer of cells ; s.g.c., sub-germinal cavity ; y., yolk. 3. Diagrammatic
surface view, a little later ; a.p., area pellucida ; a.o., area opaca ; n.p., neural
plate; p.s., primitive streak ; M., mesoderm spreading from primitive streak,
seen through ectoderm. 4. Diagrammatic surface view of later stage (end
the first day) ; a.p., area pellucida ; a.o., area opaca ; m.s., mesoblastic somites ;

р. s., primitive streak. The dark border shows the spreading of the mesodenn
over the yolk. 5. Cross-section behind the heart, at the end of the second day;
A., one of the vessels which join to form the dorsal aorta ; am., amnion fold;

с. , coelom or pleuro-peritoneal cavity of the embryo ; c'., extra-embryonic coelom ;
ep., ectoderm; hy., endoderm; s.c., spinal cord; s.g., dorsal root ganglicm
forming from neural crest ; so., somatic mesoderm ; sop., somatopleure ; spl.„
splanchnic mesoderm, with blood vessels ; spp., splanchnopleure ; N., notochord ;
m.p., myotome ; W .d., Wolffian duct, with rudiment of kidney tubule adjoining.
6. Embryo at the end of the fifth day ; a.v., auditory vesicle ; al., allantois -
C., cerebrum ; cb., cerebellium ; E., eye ; /./., fore-limb ; H., heart ; h.L, hind-
limb ; o.l., optic lobe ; pn., pineal body ; y.s., remains of stalk of cut-off yolk sac -
vag., vagus.

642

MANUAL OF ELEMENTARY ZOOLOGY

formed. As the blastoderm continues to spread, a change
comes over the lower layer cells of this area, most of which
become flattened and adhere by their edges to form a
continuous layer (Fig. 491, hyp .), the definitive endoderm
for the gut. The endoderm in the area opaca is formed,
from the lower layer cells there, somewhat later ; its cells
are cubical and have the function of taking up yolk and
passing it to the blood vessels which form in the mesoderm
above them (p. 653). A few lower layer cells remain in
the blastoccele and will join the mesenchyme (loose meso-
derm, p. 625).

The above account of the formation of the endoderm of the chick
is still the best substantiated of several that have been given. A
more recent description of the process states that the endoderm of

Pr,9r- tp.

- mes.
hyp.

Fig. 492. — A section across the primitive streak of a chick embryo.
ep., eetcderm ; hyp., endoderm ; mes., mesoderm ; pr.gr., primitive groove.

the area pellucida arises by a migration inwards and forwards of cells
from a region of the surface of the blastoderm at what will be the-
hinder end of the embryo.

While the gut endoderm .is forming, the area pellucida
is becoming pear-shaped, with the broad end forwards;
(Fig. 490, 3). As this happens, there appears in the
narrow end an opaque strip, the primitive streak ( p.s.)„
and along this a primitive groove develops. Sections
show (Fig. 492) the primitive streak is due to an immi-
gration of cells from the upper layer, which presently
reaches and fuses with the endoderm. The history of the
primitive streak of the frog (p. 620) shows that this streak:
represents the blastopore. As we shall see, the cells
which enter by it, like those which enter at the lip ol the
frog’s blastopore, form the notochord and mesoderm.

EMBRYOLOGY

643

Hyp. W’S.

A. diagram of

Before their immigration these cells lie in “ presumptive areas”
ot the surface in the same way as the corresponding cells of the frog's
embryo. The cells which will form the lateral plate mesoderm are
the hrst to immigrate; those which form the epimeres follow; those
tor the notochord enter last and lorm, at the anterior end of the streak,
a swelling, the primitive knot , which recedes (see below) as it forms
the notochord.

There are now present in the bird’s embryo all the
Gastruia. parts of a gastrula (p. 607) — ectoderm,

endoderm, archenteron (sub germinal cavity),
blastocoele, and blastopore, though the layers do not
form a sac, and though they have arisen neither by
invagination, as in the lancelet,
nor by overgrowth (epiboly), as
in the frog, but by division of a
single layer ot cells ( delamina -
tion). It should be noted, how-
ever, that in the chick there is
at no time a true blastopore,
leading from the exterior to the
enteron, and that the latter is
formed in a manner quite differ-
ent from the invagination by
which it starts in Amphioxus and
the frog.

The notochord and mesoderm
ot the embryo are
Mesoderm formed from the primitive streak, which in its
Notochord. axial part becomes the notochord (not yet
formed at the stage of Fig. 492), and laterally
grows out between ectoderm and endoderm as sheets of
mesoderm (mes. in Fig. 492 ; bounded by a dotted line in
fig. 490, 3 and 4). This transformation of the primitive
streak happens at the front end ; meanwhile the hinder
part lengthens correspondingly, and so the streak recedes.
A split which forms in the mesoderm sheets gives rise
to the coelom (Fig. 497, A), with splanchnic layer of
mesoderm within (below) it and somatic layer without
(above). As in the frog (p. 621), each mesoderm sheet gives
rise along its median side to a row of epimeres (“ mesoblastic
somites”) visible' chiefly as myotomes, sclerotomes being
indistinct, and laterally to an unsegmented lateral plate.

Fig. 493.

a section across the
primitive streak of a
chick embryo with
arrows showing the
direction of the immi-
gration of the cells which
torm the mesoderm.

Lettering as in Fig. 492.

644

MANUAL OF ELEMENTARY ZOOLOGY

Folding off.

By the end of the first day of incubation there are four or
five pairs of epimeres.

From the germ layers thus laid down the organs of the
chick arise essentially in the same way as
those of the frog. But the course of develop-
ment is much modified in detail, and has new features
added to it, owing to the differences between the eggs of
the two animals. The protected and well-provisioned egg

of a bird (or reptile) has great
advantages in that it avoids the
perils and delay of the larval stage
of an amphibian and the necessity
of visiting waters at the breeding
season ; but, as we have just
learnt, it entails forming the em-
bryo, from a thin blastoderm, which
must enclose the yolk in the course
of the development, and it also
makes necessary the provision of
special apparatus for the retention
of water, for respiration, and for
storing excreta. These things are
accomplished in the processes of
the folding off of the embryo and
the formation of the embryonic
membranes which are found in
reptiles, birds, and mammals (Fig.
494). The blastoderm continues to
extend until, at a late period

Fig. 494. — The origin of amnion and
allantois. — After Balfour.

1. Rise of amniotic folds (a. fid.) around embryo ( e ) ;
p.p., pleuro-peritoneal cavity or coelom ; y.,
yolk.

2. Further growth of amniotic folds {a. fid.) over
embryo and around yolk.

3. Fusion of amniotic folds above embryo ; a.p.,
amnion proper; s.m., false amnion or serous
membnne ; y.s., yolk sac.

4. Outgrowth of allantois ( al .) ; a.c., amniotic

cavity ; h., head end ; t., tail end.

5. Complete enclosure and reduction of yolk sac

(y.s.) ; s.m., serous membrane ; . a.p.,

amnion proper ; al., allantois ; g., gut of
embrvo.

SZ.™

c.

ent.

ep-

c.

B

Fig. 495. -^Diagrammatic sections ot the chick embryo to show the
relations of layers and folds. A. longitudinal ; B, transverse.

a.c., Anmiotic cavity ; a.f., false amnion ; a.p., true amnion ; al., allantois ; c.,
coelom ; e., embryo ; ent., enteron ; ep., ectoderm ; hyp., endoderm ; mes.,
mesoderm ; mty., mesentery : nch., notochord ; s.c., spinal cord ; s.a., sero-
amniotic connection ; v.s., yolk sac.

645

640

MANUAL OF ELEMENTARY ZOOLOGY

in incubation, it has completely enclosed the yolk. As
development proceeds, there appears in the area peliucida
a furrow that surrounds a central region in which the
embryo is forming. The furrow begins in front, in a
region, called the proamnion , which does not yet contain

Fig. 496. — A diagrammatic dorsal view of the blastoderm of a fowl's
egg towards the end of the first day (between Figs. 3 and 4 in
big. 490).

fl.0. Area °paca ; a.p., area peliucida ; a.p.', part of area peliucida into which meso-
derm has not penetrated (proamnion) ; a.v., area vasculosa ; head fold :
mes.l., anterior limit of mesoderm ; n.f., neural folds ; nch., notochord • pr sr ’
primitive groove. > j -s

mesoderm, as the crescentic head fold, and is completed
by a tail fold behind, which joins the head fold at the
sides. These folds deepen, and pinch off the little embrvo
proper from the rest of the blastoderm and the yolk around
which the latter is growing. The splanchnopleure, as it
folds inwards (Fig. 497 B), comes to form in the body a

EMBRYOLOGY

^47

tube lined by endoderm. This tube is the rudiment of the
gut. In the hinder part of the head where, as we shall see,
the rudiments of the heart are forming, the splanchno-
pleure folds in more rapidly than the somatopleure, so
that the coelom between them is very spacious, forming

Fig. 497. — Transverse sections of an early second day chick. A. A
little behind the middle of the embryo, in the region in which
folding off has not yet taken place. B. Further forward, where
the splanchnopleure is folding in under the gut and is bringing
together the two tubes whose union will form the heart.

a, o., one of the pair of vessels which will join to form the dorsal aorta ; bl.v., blood
vessels in the splanchnopleure ; c., coelom of the embryo ; c’ coelom in the
blastoderm outside the embryo ; ep., ectoderm ; eth., endothelial lining (endo-
cardium) of the heart ; h.b., hind brain ; h.m., future heart muscle ; hy., endo-
derm ; i.c.m., intermediate cell mass (nephrotome) from which the kidney

tubules and duct will be formed ; m., myotorne ; nch., notochord ; per.c., peri-
cardial cavity ; ph., pharynx ; so.m., somatic mesoderm ; sop., somatopleure ;
sp.m., splanchnic mesoderm ; spp., splanchnopleure ; Wd., position of Wolffian
duct in a somewhat later embryo.

a chamber which will be the pericardial cavity. In the
middle of the body where the splanchnopleure folds have
not yet met, the gut communicates by an ever-narrowing
stalk with a sac, the yo/k sac , which is the extra-embryonic
part of the splanchnopleure enclosing the yolk. The yolk
sac is separated from the somatopleure by a space
(c' in Fig. 490, 5 ; p.p. in Fig. 494) which is part of the

648

ft h.

fe.fi

spp.

per.c.

Fig. 498. — The chick embryo at the thirty-sixth hour of incubation. —
f rom Marshall and Hurst, Junior Course of Practical Zoology.

■A, A dorsal view ; B, a diagram of a median longitudinal section.

Lr., Rudiment of the outgrowth of the future -wall of the gut which will form the
allantois ; am., amnion, beginning to cover the head ; au., pit which will form
auditory vesicle ; f.b., fore brain ; h.b., hind brain ; hy., endoderm ; m., myo-
tome ; m.b., mid brain ; nch., notochord ; neu., pit representing the neurenteric
canal ; o.v., optic vesicle ; p.s., primitive streak ; per.c., portion of coelom
which will become pericardial cavity ; ph., pharynx ; sp.c., spinal cord ; spp.,
splancknopleure sop., somatopleure ; v., ventricle ; vit.v., vitelline vein.

EMBRYOLOGY

649

coelom, continuous with the coelom of the embryo proper,
as is seen in Fig. 490, 5. As the embryo grows 'and
lessens the yolk by absorbing it, the yolk sac becomes
smaller than the embryo.

The amnion is a peculiar membrane which envelops
the embryo. It arises in the following way.
AHantoisfnd ^ a time when the splitting of the mesoderm
into somatic and splanchnic layers has pro-
gressed some way outwards from the embryo over the
yolk there appear, starting in the proamnion, upward
folds (am. in Fig. 490, 5 ; a.fld. in Fig. 494) parallel with
the downward folds which formed the embryo, but
•consisting of somatopleure only. The folds on all sides
•of the embryo arch upwards and unite above, forming a
dome over the embryo. When their tops unite, the inner
limbs of the folds form the true amnion (Fig. 494, a.p .),
the outer limbs form the false amnion or serous membrane
•or chorion (Fig. 494, s.m.).

At the point where the folds finally meet their fusion is incomplete
and the inner and outer limbs remain connected ( sero-amniotic con-
nection).

The cavity bounded by the true amnion contains a
fluid which bathes the outer surface of the embryo ; that
between the true and false amnions is lined by mesoblast
and is continuous with the ccelomic space between the yolk
sac and the overlying somatopleure. As the split between the
layers spreads round the yolk sac, the outer layer it forms
continues the false amnion, which finally surrounds the
sac. Meanwhile the folding off of the embryo has narrowed
the connection between it and the rest of the blastoderm,
so that the amniotic cavity encloses the embryo except in
the region of this narrow umbilical stalk in the middle of
the belly. The amnion provides a bath for the embryo,
which is no more able to stand drying up than an
amphibian larva is, and, as a cushion, protects it against
injury when the egg is moved. While the amnion is being
formed, a sac known as the allantois (Fig. 494, 4, al.)
grows out from the hinder part of the gut of the embryo,
in the position in which the urinary bladder stands in the
adults of those vertebrate animals which possess it. This

650 MANUAL OF ELEMENTARY ZOOLOGY

a.a.

Fig. 499. — A diagram of the circulation of the chick at the

end of the third day.

Ihe blastoderm is viewed from below. The arteries are black, the veins in outline,

a. a., second, third, and fourth arterial arches (the first has degenerated ; a.vit.v.,
p.vit.v., veins from sinus terminalis to left vitelline vein ; ao . , dorsal aorta ;
d.C., ductus Cuvieri ; e.car., i.car., external and internal carotides ; l.vit.a.,
r.vit.a., left and right vitelline arteries ; l.vit.v., r.vit.v., left and right vitelline
veins ; s.t., sinus terminalis; s.v., sinus venosus ; v., ventricle; v.c.a., v.c.p.,
anterior and posterior cardinal veins.

The right anterior vitelline vein, which is not alwavs present, has, if it existed,
disappeared in the dwindling of the right side of the system.

EMBRYOLOGY

651

sac, like the gut Ironi which it grows out, is lined with
endoderm and covered with splanchnic mesoderm, and
projects into the body cavity. It grows down the umbilical
stalk and spreads out between the true and false amnions,
fusing by its mesoderm with the latter. In the end it
completely lines the shell. It becomes very vascular, and
by its means the embryo breathes through the porous
shell, the urine is passed into it and uric acid deposited
and retained m it, and it absorbs for the embryo water
and protein from the white.

The formation of the organs of the chick resembles in
broad outline that of the frog (compare Fig. 479
N®??ousge"y : an<^ *so-. 5 in Fig. 490). A neural (medullary)
System plate (Fig. 490, 3, n.p.) appears in the ectoderm

in front of the primitive streak : its sides grow
UP as neural folds (Figs. 496, 497, n.f.) which presently, as in
the frog, meet above the back to form the central nervous
system. The neural folds lengthen backwards, and their
lengthening keeps pace with the transformation of the
primitive streak just mentioned, so that the streak is always
behind the folds. By the end of the first day the medul-
lary folds have met in the region of the hind brain. On
the second day the front of the head begins to bend down,
setting up a cranial flexure like that of the frog, by which
the fore brain is bent down at right angles to the parts
behind it. (Fig. 49 8, f.b.) The further development of
the nervous system takes place in practically the same
manner as in the frog (pp. 622, 626).

The alimentary canal, like that of the frog, arises in
three sections, (a) The greater part is lined by
Sanaie.ntary endoderm and is known as the mesenteron.

It arises by a folding of the splanchnopleure in a
manner which has been described above (p. 646). Five
visceral clefts, which do not bear gills and of which the
hinder two pairs do not break through, arise in its anterior
portion (pharynx), (b) A small section which forms part
of the mouth is lined by ectoderm and known as the stomo-
dyeum. It arises by outgrowths which form the jaws,
surrounding a shallow depression. A hollow diverticulum
from its roof forms the hypophysis (p. 64). (c) Another

small section at the hind end, which forms the procto-

652

MANUAL OF ELEMENTARY ZOOLOGY

cue urn, is also lined by; ectoderm. The s comodaeumTreaks
through to the mesenteron at the end ot the third day,
and proctodaeum not until the fifteenth day.

I he renal organs begin to be formed during the second
half of the second day. They arise, in the
Gonads! and three sections which we have already mentioned
— pronephros (p. 439, footnote), mesonephros,
and metanephros (p. 636) — as groups of tubules formed

Fig. 500. A transverse section through the middle of the
mesonephros ot a chick embryo of 36 hours.— From Lillie.

^o. Aorta ; cot:!, coelom; col.t., collecting tubule; germ.ep., germinal epithelium
( udiment of gonad) ; glom., rudiment of glomerulus ; mst., mesentery ; n.t.
nephrogenous tissue from which tubules are formed; T. i, T. 2, T.t. tubules in
three stages of formation ; V.c.p., posterior cardinal vein ; W.d., Wolffian duct.

from the nefthrotome or intermediate cell mass ( i.c.m .
Fig. 497) which unites the epimere (“ mesoblastic somite ”)
to the lateral plate. The pronephros is never functional,
i he outer end of each of its tubules turns backwards and
joins the next one, so that there is formed a longitudinal
duct, the archinephric or segmental duct, which grows
back to join the cloaca. The mesonephric -tubules -come,
to open into this duct, and then, from the point where the
first of them joins, it is known as the Wolffian duct. The
ureter grows forwards from the hinder end of the Wolffian

EMBRYOLOGY

653

duct to receive the tubules of the metanephros (Fig. 486).
Rudiments of the gonads appear on the fourth day on the
peritoneum at the base of the mesentery (Fig. 500). At
about the seventh day sexual differentiation begins to
appear in them. Eventually the foremost part of each
mesonephros becomes in the male the epididymis and in
the female a vestige known as the epoophoron, while the
Wolffian ducts become the vas deferens in the male and
degenerate in the female. The oviducts (Mullerian ducts),
of which a pair appear but that on the right degenerates,
begin, on the fourth day, as a thickened strip of peritoneum
overlying the mesonephros. The anterior part of the duct,
is formed by the folding in of the strip. Its end then grows
back under the peritoneum till it reaches the cloaca.

Concerning the skeleton and muscles we shall not here
add anything to what has been said about the formation
of these organs in the frog (p. 629).

Blood vessels arise during the latter half of the first day

Blood vessels l°ose mesoderm (mesenchyme) just

above the endoderm of the area opaca, where
they cause the region over which the mesoderm has ex-

tended to be distinguished, as the area vasculosa , from
the region outside it (Fig. 496). They arise by clumps of
mesenchyme cells hollowing out to form blood islands in
which some of the cells become corpuscles. Later the
islands join into a network and in this presently larger
vessels differentiate. The formation of blood vessels
spreads over the area pellucida into the embryo. The area
vasculosa soon becomes bounded by a ring vessel, the
sinus terminalis. A little later there is differentiated a pair
of vitelline veins , which are continued into the embryo,
and bear thither the nutriment absorbed from the yolk.
In the embryo these join, as we shall see, below the throat
to form the rudiment of the heart ; vessels (Fig. 499,
a.vit.v., p.vit.v .) drain the sinus terminalis into them.
When the body turns to lie on its left side (p. 657) the
right half of this venous system dwindles.

The heart appears first at the beginning of the second
day. It is formed from a pair of longitudinal tubes in
the splanchnic mesoderm, each continuous behind with
one of the two vitelline veins which run in from the yolk

654

MANUAL OF ELEMENTARY ZOOLOGY

■‘ic (p. 653)* As the splanchnopleure folds in under the

Embryonic tubes join from before backwards, the

Circulation. junction eventually proceeding for some dis-
tance along the vitelline veins. As in the frog
the heart-tube is thrown into an S, constrictions mark
out the chambers, and partitions separate them. At first
there is a sinus venosus, but this later becomes merged in
the right auricle. Like that of the tadpole, it receives a
venous system with cardinal veins like that of a fish and
posteriorly is entered by the common trunk of the vitelline
vems {ductus venosus) (Fig. 501, B). Later the posterior
< ar dmals disappear, and the anterior part of the system
models itself into that of the adult. The ductus venosus
is loined on the fourth day by the allantoic vein , and a
little later by the inferior vena cava (Fig. 501, D). To-

wards the end of development the vitelline and allantoic
\etns havmg no further function, dwindle and disappear
and the interior vena cava becomes the great vein of the
hinder part of the body. The portal system is developed
Irom the ductus venosus behind the junction of the
m erior vena cava. As the heart forms there appear a
pair of dorsal aortae and then two vitelline arteries
rom the aortae to the yolk sac. Late on the second
(lay the dorsal aortae fuse midwav (Fig. eoi A) and
thereafter the junction extends backwards, ’ dinging
together the vitelline arteries, which unite. Though
there are no gills, aortic arches grow from the front
ol the heart to join the dorsal aortae. The system they
orm changes into vessels of the adult as shown in Fig.
4cS4, L. The blood which the allantois receives comes
rom the dorsal aorta by a pair of allantois arteries

,1 lg; ,5°TJ L aI-af After being oxygenated in the organ
this blood passes by the allantoic vein (Fig. 501 D al v )
t irough the portal system into the inferior vena cava. In

, ® yart; ,3° long as the ch>ck is breathing by its allantois,
the blood brought to the right auricle by the inferior vena
tv'va is not sent into the right ventricle but, by a crescentic
fold— known as the Eustachian valve— oi the auricular
wall, is directed through the foramen ovale , an opening in

hLiff "i bet^een the auricles. Thus the arterialised
d froni the allantoic vein, mixed, it is true, with some

f 1G. ^ou Plans Oi vascular systems of chick embryos. The vessels
are shown as from the ventral side. Those which are indicated
in outline have disappeared by the fifth day.

4, Arterial system on the third day ; B, venous system on the third day ; C arterial
system on the fifth day ; D, venous system on the fifth day.

2. a., Left root of the dorsal aorta, formed by the union of the aortic arches of its
side ; a.c., left anterior cardinal vein ; a. la., left allantoic artery ; al.v., allantoic
vein , hr., left brachial vein ; d.ao., dorsal aorta ; d.c., ductus Cuvieri (superioi
vena cava); d.v., ductus venosus ; h., heart; Lear., internal (dorsal) carotid
artery: t.v.c., inferior vena cava ; jugular vein ; liver; i.vit.v., left vitelline
vein ; p.c., posterior cardinal vein ; port., portal system ; s.v., sinus venosus *
v.car., ventral (external) carotid arteries ; vit.a., left vitelline artery • vit v ’
common vitelline vein. *’

655

656 MANUAL OF ELEMENTARY ZOOLOGY

tlG. 502. Semidiagrammatic views of the process of formation of
the heart in the chick. Partly after Patten.

A. Two separate endothelial tubes with thickenings of the adjacent coelomic wall
tor the heart muscles. B. The endothelial tubes are Brought together, and
aortic arches begin to grow out from their anterior ends (around the pharynx to
jom the dorsal aorta), C. The tubes unite (the muscle rudiments join around
them) and thus the heart is formed. Indications of its S flexure (p. 6S4) are
already visible.

a’a'f Aortic arches ; c., coelom ; eth., endothelial lining (endocardium) of heart ;
f.h., fore brain ; h., heart ; h.m., muscular wall of heart ; n.f., neural folds ;
ph., future pharynx ; vit.v., left vitelline vein ; Compare Fig. 497.

EMBRYOLOGY

657

venous blood from other constituents of the inferior vena
cava, reaches the left or arterial side of the heart and is
consequently distributed where it is needed. When the
chick begins to use its lungs the allantois shrivels up, the
foramen ovale closes, and the now wholly venous blood
of the inferior vena cava takes the same course as the rest
of that which enters the right auricle.

During development the posture and proportions of the
Posture. body undergo changes (Fig. 503). The pre-
cocious development of the central nervous
system is probably responsible for some of these. It causes
the head to be at first disproportionately large, the head to
suffer the cranial flexure (p. 626), which is permanent, and
the. whole body to assume a temporary ventra] curvature
which brings head and tail towards one another. As
the embryo is raised above the yolk, first the head and
then the whole body roll over to lie with the left side
downwards.

Development lasts for three weeks, and during it the
Hatching. white is taken up as additional nutriment by
the embryo, being reduced by the abstraction
of water to a solid mass, which is then absorbed by the
allantois. Finally, on the twentieth day of incubation, the
beak pierces its way into the air chamber which exists at
one end of the egg between the two shell-membranes, and
the animal begins to breathe by means of its lungs. ’ The
shrivelling of the allantois now takes place (the yolk sac
has already been absorbed), and the chick breaks its wav
out of the egg.

In all mammals except the little group of Monotremata
the egg is minute and undergoes complete and
of Mammals. nearly equal cleavage. Its development, how-
ever, is very different from that of the similar-
looking egg of Amphioxus. Instead of producing a
hollow sphere of cells which invaginates, cleavage nearly
always results in a solid, spherical mass or “ morula
and there is never an invagination, though a stage com-
parable to the gastrula arises by the establishment of
differences between layers of the cells, and possesses, in
the primitive streak, the trace of a blastopore. The later
course of development resembles in the main that of 2
42

A

A , Egg and embryo at the end of the fifth day of incubation ; B, the same at the end
of the ninth day.

a.c., air-chamber ; a.f., false amnion, united with vitelline membrane ; a.f', the same,
further united with allantois ; al., allantois ; a.p., amnion proper ; e., embryo ;
m., site of the sinus terminalis : c., she'l ; s.m., shell membrane ; w. white :
reduced to a semi-solid mass ; y.s.. volk sac.

6s«

EMBRYOLOGY

659

bird,, a yolk sac (which, however, contains no yolk), an
amnion, and an allantois being formed.

The details ot the early stages and of the formation of
the embryonic membranes differ a great deal in different
mammals. In the rabbit they take place as follows. The

Fig. 504. — Early stages in the development of the Rabbit. — After

various authors.

I. Ovum with polar bodies ; 2, two-cell stage ; 3, morula ; 4, section of a later stage ;
5, section of the young blastocyst ; 6, section of an older blastocyst ; 7, sectim!
of the embryonic area after differentiation of two layers.

■e.e., Embryonic ectoderm ; i.c., inner cells ; pr.hy., primitive endoclerm • R c
Rauber’s cells ; tr., trophoblast.

■ovum has no vitelline membrane, but is enclosed in a
Earl sta es relatively thick striated membrane, the zona
*of Mammals. radiata , and has outside that a coat of albumen

secreted by the oviduct. The morula (Fig.
5°4, 3) lies in the uterus, which is reached (p. 552) at the
end of segmentation. It is covered by a single layer of

660 MANUAL OF ELEMENTARY ZOOLOGY

cells which are rather smaller and more transparent than
those within. This layer is known as the trophoblast
(Fig. 504, 4, trl), and will form a part of the ectoderm—
that part, namely, which covers the false amnion, but not
the ectoderm of the embryo proper nor that which lines
the amnion. It now begins to absorb water and nutriment
secreted by the wall of the uterus and to be distended, so
that it separates from the inner mass of cells, except at
one side, where they remain sticking to it, at first as a
knob (Fig. 504, 5), which afterwards (Fig. 504, 6) flattens
out upon the inner side of the trophoblast, forming a
circular patch, known as the embryonic plate (or shield ,

A

B

Fig. 505. — The blastocyst on the seventh day. — After Kollikcr.

A, From the side ; B, from above.
b.hy., Boundary of the endoderm ; e.p., embryonic plate.

or area). This1 afterwards becomes oval, and will give rise
to the embryo. The bladder-like structure which has
thus arisen is known as the blastodermic vesicle or blasto-
cyst (Fig. 505). As it grows, its trophoblast cells stretch
and become thinner and flatter (Fig. 504, 7, trl). Mean-
while the inner mass cells begin to differentiate into two
layers, an outer, columnar layer of ectoderm, and an inner,
flattened layer of endoderm. The endoderm starts to
grow round the blastocyst, lining the trophoblast beyond
the embryonic ectoderm. Over the latter, the trophoblast
cells (here known as “ cells of Rauber ”) become separated
and disappear, leaving bare the embryonic ectoderm,
which at its edges becomes continuous with the tropho-

EMBR YOLOGY

66 1

blast, so that the vesicle remains unbroken. The blasto-
c\ st is now practically in the condition of the early
blastoderm of the chick, though instead of the immense
mass ot yolk of the bird’s egg there is only the fluid of
the blastocyst, and the ectoderm (including the tropho-
blast) already forms a complete vesicle. In the embryonic
plate a primitive streak and groove (where cells immigrate
from presumptive areas of the surface), medullary folds,
mesoderm, head and tail folds (separating embryo from
3 olk-sac), amnion, and allantois now arise m succession
(Fig. 5° 7)- The mesoderm, however, never extends to
the ventral side of the yolk sac, whose endoderm is
therefore, in that region, covered only by ectoderm
(trophoblastic) : eventually the ventral wall thus formed

pr9r.

,mes.

Fig. 506. A transverse section through the primitive streak.
ep., Ectoderm ; hyp., endoderm ■ mes., mesoderm ; j&r.g'r., primitive groove.

breaks up and disappears. Meanwhile, outgrowths or
41 villi ” of the trophoblast burrow into the wall of the
uterus. These are especially numerous over a thickened,
horseshoe-shaped patch of trophoblast which surrounds
the hinder part of the embryo, in the region (/A.) in which
the placenta (p. 552) will arise. After the establishment
of the membranes the embryo is known as a foetus.

f he early stages of the embryonic development of Man are im-
perfectly known, but the process appears to belong to a type which
resembles that of the chick less than the development of the rabbit
does. In it the trophoblast over the embryo does not disappear, and
the amnion is formed very early, as a cavity in the embryonic ecto-
derm, which arises as a mass of cells, not as a layer. In the floor of
this cavity the embryo is formed.

Fig. 507. — Sections of a rabbit embryo on the ninth day. — After

various authors.

A, Longitudinal section through a blastocyst removed from the uterus • B trans-
verse section of the uterus and of a blastocyst in situ. — Partly after Marshall
aL, Allantois ; am.f., amnion fold ; c., coelom ; c.p., pericardium ; c.m.. circular
muscles of uterus ; d.w.y., dorsal wall of yolk sac ; e., embryo ; h.f. head fold •
hyp., endoderm of embryo; hyp'., endoderm of yolk sac; l.m., longitudinal
muscles of uterus ; l.u lumen of uterus ; mes., mesoderm ; mm., mesometrium
of uterus m./., neural folds ; t.f., tail fold ; tr., trophoblast (grey in A, black in
u Vi •’ thlckenecl region of same in which will arise placenta ; tr.v villi of tro-
phoblast ; u.c., uterine capillaries ; v.w.y., ventral wall of yolk sac ; y.s., yolk sac.

662

EMBR YOLO GY

663

For a while the yolk sac of mammals forms a union with
The the uterine wall and through trophoblastic

placenta. villi is the main organ of nutrition and

respiration but in this it is soon replaced by the
allantois, which, as in the chick, spreads out under the
false amnion or subzonal membrane and fuses with it.
The organ thus formed is the placenta, and from it out-

Fig. 508. — Longitudinal section of a rabbit embryo on the tenth day.

am., Amnion ; am.c., amniotic cavity ; hyp"., endoderm of allantois ; y.st., stalk
of yolk sac. Other letters as in Fig. 507.

growths penetrate into the uterine wall, expanding the
original villi of the trophoblast and obtaining nourish-
ment and exchanging gases with the maternal blood in
lacunae which are formed around them by the break-
down of blood vessels in the wall of the uterus. Thus, as
in the chick, the blood in the allantoic vein is arterial,
though it has here also an important function as the
vehicle of nourishment during the greater part of em-
bryonic life. The arrangement, resembling that in the

664 MANUAL OF ELEMENTARY ZOOLOGY

chi< k (p. 654), by which this arterial blood is directed into
the left auricle, leaves traces in the heart of the adult

mm

F IG 509. — Transverse section of the uterus of a rabbit with an embryo
of the nineteenth day.— Partly after Marshall. The placenta and
membranes are now attached only by a narrow stalk to the em-
bryo, which lies on its side.

pl.t Placenta ; umb., umbilical stalk ; x, dotted line indicating position of vanished
ventral wall of yolk sac. Other letters as in Figs. 475, 476.

{Fig. 418 and its legend). The navel of the adult marks
the site ol the umbilical cord , in which the stalks and
blood vessels of the yolk sac and allantois entered the
body. The urinary bladder is formed from a remnant of

EMBRYOLOGY

665

the stalk of the allantois.
The amnion is the “ caul,”
and the placenta is shed as
the “ afterbirth.”

The resemblance between
the development

o»°voung Gnt of the minute,
stages. s yolkless eggs of
most mammals
and that of the bird’s egg,
which is large and yolky,
is a remarkable fact. It
suggests that the ancestors
of these mammals had yolky
eggs as the Monotremata
still have, that they acquired
the habit of retaining them
within the body, and that
there the allantois, which in
the bird’s egg serves prin-
cipally tor the respiration of
the embryo, enabled the
mother to make complete
provision for the nourishment

Fig. 510. — Stages in the develop-
ment of an earthworm. — After
Wilson.

1. Stage of two blastomeres ; p.c., polar

bodies (not pole cells).

2. Blastula ; M., primary or pole cell of

mesoblast.

3. Gastrula in ventral view ; Ec., ectoderm

or epiblast ; En., endoderm or hypo-
blast ; M., mesoblast.

4. Late gastrula in ventral view, showing

the bands of cells known as “ germ
bands ” : the blastopore has narrowed,
and what remains of it is now the
mouth t M ., mouth ; m., primary

mesoblast cells (pole cells) ; ms., meso-
derm bands ; N., nephridioblasts,

large cells derived from the ectoderm
which add to the bands of nephridial
cells as the embryo lengthens; Nb.,
neuroblasts, similar cells at the ends
of the bands of cells which form the
rerve cords ; n.c., nerve cords ; np.c.,
cells which will form nephridia ; si.,
stomodaeum.

666

MANUAL OF ELEMENTARY ZOOLOGY

as well as for the respiration of her offspring, and with
that the yolk disappeared. The Chordata whose embry-
ology we have been studying exemplify well the several
ways in which animals are nourished during their develop-
ment. The lancelet obtains its own food as a larva*
The frog during its early stages, and the bird throughout
development, subsist upon yolk with which the ovum
was stocked by the mother. Birds are provided also
with nutriment (the “ white ”) around the ovum in the
shell. Mammals are nourished directly from the mother’s
body both before and after birth.

Fig. 51 l- — Three stages in the segmentation of the egg
of the crayfish.— From Parker and Haswell.

nil., Nuc lei ; y.p., “ yolk-pyramids.”

With the embryology of invertebrata we can only deal
very briefly.1 The development of Hydra and
Pn?ertebrates.°f Obelia has already been described (pp. 220 and
228). It includes complete and equal cleav-
age of the ovum, a hollow blastula, the conversion of this
into the two layered (gastrula) stage by the immigration,2
of some of the cells of its wall, the origin of the enteron as
a split in the mass of endoderm, and the shaping of the
gastrula into the adult by a simple process. In the starfish
cleavage is complete and equal and ends in the establish-
ment of a blastula, not unlike that of the lancelet, from

1 The external features and habits of the larvae and other young
stages of invertebrata have been described with those of the adultsf

Compare p. 643. The modes of formation of the gastrula
(“ gastrulation ") of animals are : invagination or “ emboly,” epiboly,
delamination, and immigration.

EMBR YOLO GY

667

which by invagination a gastruia is formed. The meso-
derm arises from the anterior end of the archenteron, as a
pouch whose cavity gives rise to the coelomic spaces.

In the earthworm (Fig. 510) and swan mussel cleavage of
the ovum is complete but unequal, and forms a hollow
blastula, which invaginates to give rise to a gastruia.
I he mesoderm arises along the ventral side as two bands,,
each formed by the division of one of a pair of pole cells at
the hind end which are derived from a cell “ determined

Fig. 512.— Part of a longitudinal section through the egg of a cray-
fish after the enteron has been established and the blastopore has
closed. — From Tarker and Haswell, after Reichenbach.

^/.(Ectoderm ',e*id., endodertn ; ent., enteron; mrs., mesoblast; ficdm., proct®
daum (for hind gut); std»i., stomodaeum (for fore gut); th.ab., rudiment from
which abdomen and part of thorax arise ; yk., 3’olk, lying at this stage in what
is morphologically the blastocoele.

for their formation during cleavage, and in the earthworm
each band subsequently divides into a row of mesoblastic
somites. These as they form become hollow, and the
cavity of each unites with that of its fellow on the other
side of the body to become the coelom of one of the seg-
ments of the adult. In the crayfish the cleavage (Fig. 511)
is incomplete, but, as will be seen from the following
account, it is of a different kind from the incomplete
cleavage of the ova of the chick and dogfish. The nucleus
divides till a number of daughter nuclei are formed, and
these migrate to the surface, where they are at first

668

MANUAL OF ELEMENTARY ZOOL^OGY

embedded in a continuous sheet of protoplasm which
encloses a central mass of yolk (“ centrolecithal ” cleavage).
Afterwards this protoplasm divides into cells which
constitute a one-layered blastoderm enclosing the yolk
mass. Thus there arises a sort of blastula which has no
blastocoele, but contains yolk. A shallow invagination
on one side of this, pushing into the yolk, gives rise to a.
gastrula with a small enteron (Fig. 512). The mesoderm
arises as two ventral bands, _ though pole cells are not
ound : it forms mesoblastic somites, whose cavities
afterwards disappear, save in the segment of the antenna
where they become the end-sacs of the green glands. The
later development of these animals cannot be followed
here, but it may be stated that it is quite unlike that of
the Vertebrata.

A comparison of the processes that have just been
von Baer’s Law. described shows two facts of importance.

.M They all have certain features in common.
A) le animals which are more alike as adults resemble
one another longer in development. This generalisation is
known as von Baer's Law. All animals have at one stage
a single nucleus. All Metazoa pass later through a gastrula
stage of two layers only. When the Triploblastica acquire
the third layer, Annelida, Arthropoda, and Mollusca have
111 common a process in which it starts as ventral bands,
while in Chordata it arises (after immigration) upon the
dorsal side of the primitive gut. All Chordata have also
at one stage a notochord, a hollow dorsal nervous system
torm^ by the folding of a neural plate, and visceral clefts.1

ertebrata have at a later stage a cartilaginous
skeleton and a circulatory system like that of a fish. At
a later stage still, the frog, bird, and mammal have
pentadactyle limbs and the rudiments of lungs. The
embryo of a rabbit is at one stage much like that of
many other mammals, then it takes on the features of a
rodent, finally it shows those of its own species. At the
same time it must not be overlooked that von Baer’s law
holds good only m a very general sense. The resemblance
between the young stages of related animals is never exact.

in teram;>nfCleftnS ' nshou]d not be aPPked to these unless, as

in nshes and tadpoles, they bear gills.

EMBRYOLOGY

669

and is often greatly obscured by disturbing factors, such as
variations in the amount ot yolk present or the precocious
development of certain organs. Thus, tor instance, the
two-layered stages of the lancelet, frog, and chick (Figs.
462, 472, 490, 2) are extremely unlike on account of
differences in the amount of yolk they contain ; and again
amnion and allantois, which are peculiar to reptiles, birds,
and mammals, are developed at an exceedingly early
stage, when the embryo is only beginning to take on the
features which are common to all chordate animals.

Regulating
Factors :
i. Chromo-
somes and
Cytoplasm.

From the preceding pages of this chapter, the student
will have learned what a complicated process
is the development of the ovum into the
adult individual. As to how this process
is caused to pursue truly its intricate course,
we have as yet very incomplete information,
but in the main it appears to be regulated in the
following way. The tact that the fertilised ovum
grows into the likeness of its parents is due in the
first place to the possession by its nucleus of chromo-
somes derived from each of them, and often, perhaps
always, also to certain qualities ot the maternal cytoplasm
in it. But, in the divisions by which the cells of the body
are formed, each cell receives a complete set of chromo-
somes, and has therefore, so far as chromosomes are con-
cerned, the potentiality of assuming the features peculiar
to any part ot the body. Sometimes, though by no means
always, the cytoplasm appears to be equally indifferent.
What is it that decides which potentialities shall be realised
in each of the cells ?

In the mass of cellular raw-material certain external
factors establish the main lines of the lay-out
the^Lay^out °f ^ie body, d he principal axis, which in
bilateral animals lies fore and aft, is usually
set up in the ovum before it is fertilised, by influences to
which it is subjected in the ovary of the mother. In the
frog, for instance, the anterior end is located on that side of
the ovum which is towards the surface of the ovary, and, as
in many other cases, the yolk is laid down in the hinder part,
leaving in the anterior part unencumbered protoplasm for
the formation of the delicate nervous and sensory organs.

670

MANUAL OF ELEMENTARY ZOOLOGY

The nucleus, . though it can only form frog nuclei, has in it
the potentialities of all the nuclei of a frog. The cytoplasm
is not only predetermined to be frog cytoplasm, but is
already under the influence of factors which decide that
part of it shall be head-end cytoplasm and part tail-end
cytoplasm. The dorsal and ventral sides are most often
determined at fertilisation ; in the frog, for instance, the
side opposite to the point of entry of the spermatozoon
becomes dorsal.

Other factors, limiting the capabilities of the cells in the

iii organism-* Several reSions> determine the nature of the
Factors. * organs to be formed m each locality of the body.

0) In some cases these factors have already
arisen m the ovum, where parcels of cytoplasm sometimes
distinguishable by colour, exist, and are marshalled by the
divisions of cleavage until they come to lie in the regions
whose capacity for organ-formation they determine. It is
even possible, for instance, in certain molluscs, to cut off
portions of the ovum and to find in consequence certain
parts of the normal body missing in the larva. Regions so
predestined m the ovum are held to contain specific
■organ-forming substances. Naturally in -these cases the
blastomeres into which the ovum is" cut up at cleavage
contain different complements of organ-forming substances,
and are thus already specialised for forming certain organs
and incapable of giving rise to others. The cleavage of
such an organ is called determinate cleavage. It is found,
for instance, in annelids and molluscs and in Amphioxus
but not m echinoderms and only to a limited degree in
\ ertebrates. ( b ) In animals in which the cleavage is
indeterminate , there is at first no limitation of the oro-an-
forming potentialities of the cells. Each cell can assume
any form proper to the species, according to its circum-
stances. If the first two blastomeres of an amphibian be
separated, each will form a complete half-size embryo
1 his means, of course, that portions of each blastomere
v uch under normal conditions would have formed certain
organs actually give rise to quite different structures.

I his is shown very strikingly in the formation of partial
twins by incompletely separated blastomeres, part of each
first blastomere behaving normally and part abnormally.

EMBRYOLOGY

671

When cleavage is completed, groups of cells transplanted,
for instance, from the position of the eye and grafted into
the side of the trunk will there give rise to ordinary
epidermis. But at a later stage determination sets in,

Fig. 513. — Potentialities of the blastomeres of Amphibia.

Left : Result of separating the first two blastomeres by a fine hair ; two larvae
are produced. Right : Result of partially dividing a segmenting ovum ;
a two-headed monstrosity is produced.— From Wells, Huxley, and Wells,
The Science of Life, after a drawing by L. R. Brightwell.

and now the fate of any group of cells is unchangeable.
The eye rudiment, or that of a leg, if transplanted, will
give rise to an eye or a leg and to nothing else, wherever
it be placed and however useless it would be in its new
position. This determination takes place under the in-

672

MANUAL OF ELEMENTARY ZOOLOGY

flaence of an entity known as an organiser . In amphibia
the dorsal lip of the blastopore assumes this function. Bv
an influence which is not fully understood but is probably
exercised by the emission of chemical substances, it limits
the potentialities of the hitherto indeterminate cells of
the embryo in accordance with a plan which is that of the
future body. It will exercise this influence even if it be
glutted into another embryo, so that the latter comes to
contain a second set of organ rudiments (Fig. 514). Its

-’V •-•••;;

. &

dominance of the
body recalls that
which we have
noted in the apex
ot a hydroid

colony (p. 233)
Some of the rudi-
ments established
by the primary
organiser will
develop without
turther external
influences ; the
Fig. 514.— A section through a frog embryo optic cup is an
mto which has been grafted the dorsal instance of this *

lip of the blastopore of another embryo, sucp rudiments
and which has consequently developed a rudiments

seminrl ct*t- nf y are said to be

self - differ entiat-
i n g. M u c h
differentiation,

, . , however, is de-

pendent upon secondary organising influences exercised
by rudiments already established ; thus, for instance, the
lens of the eye is caused by the optic cup to develop from
the overlying ectoderm.

The developing rudiments, if they are to form normal
iv. Function. organs, require the stimulus ot function. The
. effec t ot use and disuse upon the size and

f CIency of organs in post-embryonic organisms is well
known. This effect, which is most familiar in the case of
muscles but is exercised equally elsewhere, is a very
important factor in securing the normal development of

— * wwiisccjucnuy u<

second set of organ rudiments.

br.Med., proper nerve cord of the larva ; sec. Med
bram induced by the graft; l.sec.lab., left in-
duced ear-vesicle.— From Haldane and Huxlev.

EMBRYOLOGY

673

organs in the early stages of life. Nervous, muscular, glandu-
lar, and particularly the skeletal tissues are susceptible
to it, and their response affects the organs they compose.

Finally, the development of the several parts of the
V. Hormones, body in proper proportions and at the right
times is largely due to the liberation of hor-
mones and to the specific reactions of the rudiments to
them. The anterior lobe of the pituitary body produces

Fig. 5 15.- — Two dogs of the same litter : from that on
the left the anterior lobe of the pituitary body was
removed at the age of eight weeks ; that on the right
was allowed to grow up normally. — From Schafer.

a hormone which, among other functions, stimulates the
growth of bone. Excess or defect of it produces giants
or dwarfs. The substance thyroxin which is secreted by
the thyroid gland and has a stimulating effect on meta-
bolism (p. 63) is of great importance to the development
of vertebrates. Lack of it, owing to imperfection or
removal of the glands, produces defective individuals.
Human children of this kind are known as cretins. They
are stunted, pot-bellied, ugly, and of imperfect intelli-
gence. This condition can be completely cured by
43

674 MANUAL OF ELEMENTARY ZOOLOGY

administering extracts of the thyroid gland. In the
amphibia, the metamorphosis of the tadpole is brought
on by the secretion of the thyroid. If the thyroids of a
tadpole be cut out, the larva will not turn into a frog,
though it continues to grow and reaches a large size
(Fig. 33). On the other hand, feeding young tadpoles

A B

Fig. 516. — Thyroid action.

A, A cretin, 23 months old. B, The same child, 34 months old, after
administration of sheep’s thyroids for 11 months. — From Starling,
after Osier.

with thyroid tissue brings on early metamorphosis and
produces minute frogs. The testes and ovaries of verte-
brate animals produce hormones which bring about the
development of the secondary characters of sex. The effect
ot the lack of male hormones in castrated animals such
as oxen and capons is well known. By means of hor-
mones various organs of the body time one another during
development.

CHAPTER XXVIII

CLASSIFICATION

The animals that we have examined in the foregoing

Classification : ^aJ)lers bave been chosen with a view to
iuecies* meir serving, among other things, as examples
.the principal kinds of creatures that con-
stitute what is known as the Animal Kingdom Ex-
plicitly or implicitly, the study of different objects of
any kind must always proceed by a recognition of their
resemblances and differences, but the number of
dnferent kinds of animals is so enormous that it is quite
impossible to study them without arranging them in an
orderly classification according to their degrees of likeness.
We have seen that no two individual animals are wholly
ahke. The offspring of any parent are always unlike k
and unlike one another. Even so-called “ identical twins ”
which deserve that name at their birth, become to some
degree different by the different action of their surroundings
upon them as they grow up. Heredity, in fact, does not pro-
duce absolute resemblance, but is qualified by what, using the
term in its widest sense, we may call variation , whether it be
due to an unlikeness in the offspring at birth or acquired by
the impress of the surroundings during their lifetime. At the
same time, the likeness between the offspring of any parent
is, on the average, greater than their likeness to individuals
descended from other parents, and in this fact we find
the first degree of resemblance between animals. For
practical purposes however, the resemblance between
members of a family (in the ordinary sense of the word)
is useless m classification, on account of the vast number

of small divisions it gives and the impossibility of identi-

675

676

MANUAL OF ELEMENTARY ZOOLOGY

fying them. A more practicable basis is found in the fact
that animals which are closely alike will breed together and
give fertile offspring, whereas those which are less alike
will not. Thus the offspring of two horses is fertile, but
that of a horse and an ass is not, while breeding between
horses and oxen is impossible. The primary groups of
zoological classification consist of individuals which breed
together, but not with individuals outside the group to
give fertile offspring, or of which it is concluded from
their likeness that they do so. Such a group is known as
a species. We have seen examples of the kind of differences
which separate species in the case of the hares and rabbits
(p. 520), of the crayfishes (p. 287), of the Hydras (p. 208),
and of the Entamcebce (p. 179). It is believed that all the
members of a species are united by blood kinship ; that
is to say, that they are all in the long run the descendants
of one pair or several related pairs of parents, so that their
relationship is only an extension of that which exists
between offspring of the same parents. Thus the exclusive
resemblance between the members of a species depends on
two things : (1) their community of descent, (2) the fact
that their common inheritance is not weakened by inter-
breeding with unlike kinds of animals. At the same time
it must not be overlooked that upon the average, two
members of a species differ in more respects than two
children of one parent.

Although the things that are denoted in Biology by the term
“ species ” are natural entities, the term is hard to define. A species
is a group of individuals which (a) have in common morphological
or physiological features (and usually both) which they do not share
as a whole with other groups, (b) are fully fertile inter se, (c) do not
produce fully fertile offspring and usually produce none, with in-
dividuals outside the group. Two groups, each possessing the
requisite resemblances inter se and fertile inter se, may normally
have the third requirement for specific distinctness — that they do
not breed together — but be capable in certain circumstances
(e.g. in captivity) of breeding together to produce fully fertile offspring.
In such a case, the lack of interbreeding may be due to one of several
causes — to difference of breeding seasons,, to psychological factors,
to ecological, or to purely geographical separation. Whether two
such forms are to be regarded as distinct species or as subspecies of
a single species depends upon the judgment of systematists, which
will usually be based on the degree of difference between them.

CL A SSIFICA TION

677

Higher

Groups.

Species are grouped together by zoologists into divisions
of a higher grade known as genera. A genus
consists of several species which resemble one
another closely, but its limits are determined
by convenience only, and are not natural, like those of
a species. To every species there is assigned a Latin
name consisting of two words, of which the first denotes
the genus to which the species belongs, while the second
is peculiar to the species. Thus the generic name of the
rabbits and hares is Lepus , the specific name of the rabbit is
cuniculus , the common hare is Lepus timidus , the mountain
hare L. variabilis. The names of the species of Astacus ,
Hydra and Entamoeba have already been given. The
Latin names of many species are arbitrary, and some are
even misleading, but they have the advantage of providing
a generally recognised, international nomenclature. In the
foregoing pages the Latin name of each species has been
given. Above the genus are many divisions of the same
nature, but higher rank. Genera are grouped into
families , these into orders , orders into classes , classes into
phyla , and in many cases it has been found necessary to
institute additional grades of division, such as subclasses,
subphyla, etc. The systematic position of the frog will
serve as an instance of this arrangement. The frog
is the Species R. temporaria , of the Genus Rana , Family
Ranidce , Order Anura , Class Amphibia , Subphylum
Vertebrata , Phylum Chordata , Grade T rip l oblastica ,
and Subkingdom Metazoa. The following Table
shows the main lines of the classification of the animal
kingdom : — -

I. Subkingdom Protozoa.

Animals whose bodies have not a cellular structure.

Contains only the :

Phylum Protozoa.
a. Class Mastigophora.

Protozoa which move by means of flagella.

e.g. Chlamydomonas , Polytoma , Euglena ,
Peramena , Copromonas , Trypanosoma , the
Choano-ftagellata.

6;8

MANUAL OF ELEMENTARY ZOOLOGY

b. Class Sarcodina.

Protozoa which move by means of pseudopodia,
e.g. Amoeba , Entamoeba , Pelomyxa.

c. Class ClLIATA.

Protozoa which move by means of cilia and have
usually nuclei of two kinds.

e.g. O palina, Paramecium , Balantidium y
Nyctotherus , Vorticella , Carchesium .

d. Class Sporozoa.

Protozoa, usually with no external organs of
locomotion, which are always internal parasites
and form numerous spores.

e.g. Monocystis , Plasmodium .

II. Subkingdom Parazoa.

Animals whose bodies are composed of semi-
independent cells, some of which are choano-
cvtes. Contains only the :

Phylum Porifera.

e.g. Sycon , Leucilla , Euspongia.

Iff. Subkingdom Metazoa.

Animals whose bodies contain specialised and
subordinate cells, none of which are choano-
cytes.

A. Grade Diploblastica.

Metazoa in whose bodies there are only two
protoplasmic layers, ectoderm and endo-
derm. Contains only the :

Phylum C CEL ENTER AT A.

Radially symmetrical, diploblastic animals.
The most important members of this
group are :

Class Hydrozoa.

Polyps and medusae with ectodermal gonads
and no vertical partitions in the enteron.
e.g. Hydra , Obe/ia.

CL A SSIFICA TION

679

Class Scyphozoa (Acalephze).

Medusae with endodermal gonads and with
vertical partitions in the enteron of the
polyp stage when the latter exists,
e.g. Aurelia.

Class Anthozoa.

Polyps without a medusoid generation and
with endodermal gonads, a gullet, and
vertical partitions in the enteron.

Here belong Sea Anemones and Corals.

B. Grade Triploblastica.

Metazoa in whose bodies a third layer, the
mesoderm, lies between ectoderm and
endoderm and usually contains spaces
known as the hcemocoele and coelom.

1, Phylum Platyhelminthes.

Flat, worm-like Triploblastica without anus,
blood vessels, or (in the adult at least)
body cavity, with an excretory system
formed of branched tubes ending in flame
cells, and with a complicated, usually
hermaphrodite system of reproductive
organs.

a . Class Turbellaria.

Free-living, ciliated Platyhelminthes.
e.g. Planar ia.

b. Class Trematoda.

Parasitic Platyhelminthes with a cuticle, a
forked gut, and no proglottides,
e.g. Distomum , Schistosoma .

c. Class Cestoda.

Parasitic Platyhelminthes with a cuticle, no
gut, and proglottides which break off from
the body.

e.g. Tcenia.

68o MANUAL OF ELEMENTARY ZOOLOGY

2. Phylum Nematoda.

Bilaterally symmetrical, unsegmented Triplo-
blastica, with an anus, an intracellular
body cavity, a stout cuticle, and no limbs.

e.g. Anguillula , Tylenchus , Filaria ,
Trichinella , Asians, Oxyuris , etc.

The following phyla and others which possess a
coelom are known as Ccelomata.

3. Phylum Annelida.

Bilaterally symmetrical, segmented Triplo-
blastica, with a closed blood-vascular
system, a well-developed coelom, nephridia,
a double ventral nerve cord, parting in front
to enclose the gut, and a thin cuticle.

a. Class OligochJeta.

Annelida without parapodia, with chaetae.
e.g. Lumbricus .

b. Class P OLYCHAETA.

Annelida with parapodia and numerous
chaetae.

e.g. Nereis , Arenicola.

c. Class Hirudinea.

Annelida without parapodia or chaetae, with
two suckers, and with canalicular coelom,
e.g. Hirudo.

4. Phylum Arthropod a.

Bilaterally symmetrical, segmented Triplo-
blastica, with an open blood-vascular
system, a very restricted coelom, a double
ventral nerve cord parting in front to
enclose the gut, a thick cuticle, and paired
jointed limbs, some of which serve as jaws.

a. Class Crustacea.

Aquatic , Arthropoda with two pairs of
antennae, and usually with gills,
e.g. Astacus , Cyclops.

CLASSIFICA TION 68 1

h . Class Hexapoda or Insecta.

Land Arthropoda without gills, but with in-
ternal air tubes for breathing, with one
pair of antennae, three pairs of legs, and
usually two pairs of wings.

The characteristic features of the following groups
of insects and examples of their members are given
in Chapter XVI.

Sub-class Ametabola (Apterygota).

Sub-class Heterometabola.

Orders : Orthoptera, Odonata. Hemiptera.

Sub-class Holometabola.

Orders : Coleoptera, IIymenoptera, Diptera,

Aphaniptera, Lepidoptera.

c. Class Myriapoda.

J apid Arthropoda without gills but with in-
ternal air tubes, with one pair of antennae,
numerous pairs of legs, and no wings.

Centipedes and Millipedes.

d. Class Arachnida.

For the most part land Arthropoda without
gills, but with internal air spaces ; and all
without antennae and with four pairs of legs.

i. Order Scorpionida.

Arachnida with segmented abdomen, bear-
ing a poison sting in the telson, chelate
pedipalpi, lung-books, and no spinnerets.

Scorpions.

ii. Order Araneida.

Arachnida with unsegmented abdomen,
poison glands in the chelicerae, pedipalpi
not chelate, lung-books and spinnerets.

e.g. Epeira , Tegenaria (the House
Spider).

iii. Order Acarina.

Arachnida with unsegmented abdomen,
pedipalpi not chelate, and no lung-books
or spinnerets.

e.g. Demodex , Sarcoptes , Ixodes.

682

MANUAL OF ELEMENTARY ZOOLOGY

5. Phylum Mollusca.

Bilaterally symmetrical, unsegmented Triplo-
blastica with an open blood-vascular
system, a perivisceral coelom of moderate
size, a nervous system which encircles the
forepart of the gut, a shell, but no cuticle,
a mantle fold, and a ventral, muscular
foot.

a. Class Lamellibranchiata.

Mollusca with bivalve shell, compressed
body and foot, plate-like gills, and no
head or radula.
e.g. Anodonta.

b. Class Gastropoda.

Mollusca with shell, it present, in one piece^
depressed body, twisted visceral hump,
flat-solecl foot, leather- or comb-like gills,
a head, and a radula.

e.g. Helix , Buccinum (a whelk).

c. Class Cephalopoda.

Mollusca with shell usually internal or
absent, if present always in one piece,
body appressed from before backwards,
foot converted into a funnel and tentacles
round the mouth, feather-like gills, a head,
and a radula.

e.g. Octopus , Sepia, Nautilus .

6. Phylum Echinodermata.

Radially symmetrical, marine Triploblastica
without true blood vessels, with a spacious,
complicated coelom, and with calcareous
plates in the dermis.

Aster las, Sea-urchins, Sea-lilies, etc.

7. Phylum Chordata.

Bilaterally symmetrical, usually segmented
f riploblastica, with a closed blood-vascular
system, a spacious coelom, a hollow, dorsal
central nervous system, a notochord, and
gill-clefts (visceral clefts).

CL A S SILICA TION

683,

A. Subphvlum Cephalochorda.

C hordata with a notochord which runs from
end to end of the body and lasts through-
out life, an atrium, and very numerous
gill-clefts provided with tongue-bars ; with-
out definite brain, and without heart, limbs,,
or skeleton of bone or cartilage,
e . g. A mphioxus.

B, Subphylum Vertebrata.

Chordata in which the notochord does not
reach the front of the head and is usual lv
reduced or lost in the adult, without
atrium, with few visceral clefts, which are-
without tongue-bars and are often lost
in the adult ; with well-developed brain,
heart, usually two pairs of limbs, and
always an internal skeleton of bone or
cartilage.

a. Division Cyclostomata.

A ertebrata without jaws, whose hypophysis-
retains its opening.

e.g. Petromyzon.

IS, Division Gnathostomata.

^ ertebrata with jaws, whose hypophysial*
opening closes.

a. Class Pisces.

Cold-blooded Gnathostomata with paired fins.,
bony scales, rays in the median fins, per-
sistent gill-clefts, and no lungs, amnion, or
allantois.

i. Subclass Aphetohyoidea.

Extinct fishes in which the mandibulo-hyoid
cleft was not reduced, the jaws were auto-
stylic, and the skeleton contained bone.

ii. Sub-class Elasmobranchii.

Cartilaginous fishes with reduced mandibulo-
hyoid cleft, without an air-bladder,
e.g. Scy Ilium, Raia.

MANUAL of elementary zoology

iii. Sub-class Actinopterygil

Fishes with reduced mandibulo-hyoid cleft,
bone in the skeleton, an air-bladder not
used as a lung, and no internal nares.

(1) “ Ganoid ” Orders (p. 466).

(2) Order Teleostei.

Actinopterygii without spiracle, spiral
valve, cloaca, or conus arteriosus.

e.g. Salmo , Gadus , Pleuronectes , and
most fishes.

iv. Sub-class Choanichthyes.

Fishes with reduced mandibulo-hyoid cleft,
bone in the skeleton, internal nares, and
air-bladders used as lungs.

(1) Order Crossopterygii.

Choanichthyes with normal upper jaw
and separate teeth on jaw margins.

(2) Order Dipnoi.

Choanichthyes with strong crushing bite
owing to fusion of upper jaw bar with
cranium and union of teeth into com-
pound structures on the palate,
e.g. Lepi do siren*

b. Class Amphibia.

Cold-blooded Vertebrata with pentadactyle
limbs, usually no scales, no rays in median
fins, lungs, shell-less eggs, no amnion or
allantois, and a tadpole larva with gill -
clefts which are usually lost in the adult.

i. Order Stegocephali.

Extinct amphibians, with a complete covering
of dermal bones on the skull and often a
dermal armour on the body.

e.g. Capiiosaurus.

CL A SSIFICA TION

6S5

ii. Order Gymnophiona.

Amphibians of worm-like habits, without
girdles, limbs, or tail, and with rings of
small scales in the dermis.

e.g. C '(E cilia.

iii. Order Urodela.

Amphibians with girdles and usually short
limbs, with tail and no exoskeleton.

e.g. Mo/ge, Salamandra.

iv. Order Anura.

Amphibians with stout bodies, long legs, no
tail, and no exoskeleton.

e.g. Rana.

c. Class Reptilia.

Cold-blooded Vertebrata with pentadactyle
limbs, horny scales, no median fins, lungs,
large, heavily yolked eggs laid in calcareous
shells, no larva, an amnion and an allantois
in the embryo, and the visceral clefts never
provided with gills.

The characteristic features of the following Orders of
Reptilia and examples of their members are given in
Chapter XXIV.

Lacertilia, Ophidia, Chelonia, Crocodilia,
Rhynchocephalia.

d . Class Aves.

Warm-blooded Vertebrata with pentadactyle
limbs, of which the first pair are wings,
feathers, horny scales on the legs, no
median fins, lungs, large, heavily-yolked
eggs laid in calcareous shells, no larva,
an amnion and an allantois in the embryo,
and the visceral clefts never provided with
gills.

e.g. Columba.

*586

MANUAL OF ELEMENTARY ZOOLOGY

e. Cl ass M AM MAI. I A.

Warm-blooded Vertebrata with penta-
dactyle limbs, hair but no scales, median
fins only in some whales, where they have
no rays, lungs, eggs almost always minute,
developing within the mother, milk-glands,
no larva, an amnion and an allantois in the
embryo, ■ and the visceral clefts never pro-
vided with gills.

The characteristic features of the following groups of
mammals are stated and examples of the members
mentioned in Chapter XXV.

i. Sub-class Protqtheria,

Order Monotremata.

ii. Sub-class Metatheria.

Order Marsupialia,
iii. Sub-class Eutheria.

Order Cetacea.

Order Edentata.

Order SlRENIA.

Order UNGULATA.

Sub-order Proboscidea.

Sub-order Hyracoidea.

Sub-order Artiodactyla.

Tribe Suina.

Tribe Ruminantia.

Sub-order Perissodactyla.

Order Rodentia.

Order Carnivora.

Order Insectivora.

Order Chiroptera.

Order Primates.

Sub-orders: Lemuroidea, Anthropoidea.

CHAPTER XXIX

EVOLUTION

From the earliest days ot the science two theories have
Evolution, been current as to the origin of the differences
between the several kinds of animals. One
contented itself with the statement that each species has
come into being independently by a process of special
creation , whose method it did not attempt to explain.
The other, which is now held by all zoologists, alleges
that every species has sprung from some other species
that was in existence before it, by a process known as
evolution , which starts from some of the differences
(u variations ”) which exist between parents and their
offspring, and that xhe differences upon which the zoologist
founds genera and higher groups are due to the unlikeness
between species being increased by the same process. The
theory of evolution may be stated, in the words of Charles
Darwin, as follows : All the living forms of life are the
lineal descendants of those which lived long before the
Cambrian epoch ; we may feel certain that the ordinary
succession by generation has never once been, broken.”
Darwin adds : “ There is a grandeur in this view . . . that,
from so simple a beginning, endless forms most beautiful
and most wonderful have been evolved.” Evolution is an
alteration of the average characters, either of the whole of
a species or of a certain group of its members, from genera-
tion to generation in a constant manner, by which they
become so. different from what they were at first that a new
species arises. It it take place only in a group of the
members of a species, the other members continuing to

propagate their kind unchanged, or if it proceed in different

687

688

MANUAL OF ELEMENTARY ZOOLOGY

#

Charles Darwin.

Bora 1809, died 1882.

[Fig. 517.]

EVOLUTION

689

directions in several groups at the same time, and be
attended by infertility between the groups, then the number
ot species will be increased. The dying out of a species,
v hich sooner or later practically always happens, removes
the link which unites any descendant species it may have
had, both to one another and to the ancestral species from
which the parent species was itself descended. Thus the
existing species of living beings are like the live tips of
the branches ot a tree which is being killed trom the root
upwards, d he general tendency of evolution is to increase
complexity of organisation, that is, to produce higher
organisms from lower ones, though sometimes, notably
in the production ot parasites, it brings about simpli-
fication.

I he theory ot evolution is based upon several classes of
evidence. (1) It is supported by the facts upon
Ivolutitm which classification is based. Species, genera,
Classification, families, orders, etc., are like the branches of a
genealogical tree, and when they are arranged
as such suggest strongly that they have arisen by modi-
fication, each from the preceding grade. By the alteration
in different directions of groups of members of a single
species, the several species of a genus would arise. As
each of these pursued its own line of evolution it would
become^ more unlike its congeners until it reached the
rank of. a genus, by which time it would generally have
given rise to species of its own, and so forth. Every
attempt to classify animals results in an arrangement
which to some extent suggests the evolution of its members,
but in modern zoology classifications are expressly so
constructed as to show what are believed to have been
the lines of evolution which animals have followed. Each
of. the groups of such a classification represents an
original species, from which all the sub-divisions of the
group are supposed to have arisen by descent with modi-
fication in various directions. As an illustration of this,
the several groups of the classification of the Mammalia
given in Chapter XXV. may be arranged in the form of
a genealogical tree as in Fig. 518.

(2) The facts ot morphology also support the theory of
evolution. In our survey of a series of types of animals
44

690

MANUAL OF ELEMENTARY ZOOLOGY

we have seen how organs which serve different functions
Morphology are °*'ten built upon the same general plan,
which is modified in different directions in
the several instances. Thus the fore limbs of a frog, a
rabbit, a man, a horse, a whale, and a bird are all built

J' IO. 518. — The classification of the Mammalia arranged in the form
of a genealogical tree. The Ungulata are arranged according
to the usual, but rather old-fashioned, classification adopted in
the text (p. 575), It would be more correct to make the artio-
dactyle branch start from the base of that which bears the Carni-
vora. The Sirenia should be nearer to the Proboscidea. N.W.
and O.W. in the top line stand for the New-world and Old-world
groups of monkeys.

upon the pentadactyle plan (p. 49), though the parts are
ot different shapes in the several cases, and certain of the
bones may be missing or fused. It is difficult to find any
satisfactory explanation of this except the evolution of
the animals in question from a common ancestor whose
fore limbs were of the type which they share. Organs
which are believed thus to have arisen by modification

Class Subclasses Orders Suborders Tribes

E VOL UTION

691

of identical organs in an ancestral animal are said to be
.homologous} Thus the wings of a bird and a bat are
homologous with one another and with the hands of a
man and the paddles ot a whale. Organs which have

1- IG. 519. — A dissection of the human ear to show the useless
vestigial muscles. — From Gray.

the same function, but have arisen independently, are
called analogous . The wings of a bird and an insect or

The term is extended to include the case of members of a series
in one individual, such as the nephridia of an earthworm or the legs
01 a cockioach, which are said to be serially homologous because
they are built upon the same plan, so that the repetition of structure
which is seen in them appears to be of the same nature as the re-
petition of the structure of an ancestor in its descendants.

692

MANUAL OF ELEMENTARY ZOOLOGY

the legs of a rabbit and a crayfish are analogous, since
they perform similar functions but are built upon radically
different plans. In the same class of evidence we may place
the existence of vestigial organs , such as the remains of the
pelvic girdle of whales, and the tail-bones and ear muscles
(Figs. 449, 519) of man, which can only be satisfactorily
explained on the supposition that they were functional in
an ancestor.

(3) The facts of embryology suggest evolution. We

have seen that different animals pass through
Em ryoogy. s’mpar stages, and that animals which are

more alike resemble one another longer during develop-
ment. All animals have at one stage a single nucleus for
all purposes, all Metazoa have at a later stage two layers
only, all Vertebrata at a still later stage have visceral
clefts and a notochord, all mammals at a later stage yet
are five-fingered, and so forth (see p. 668). The simplest
explanation of these facts is that it is due to the repetition
during the development of an animal of characters which
appeared during the development ot its ancestors, the vary-
ing duration of resemblance during development being
due to various degrees of relationship by descent.

This explanation of von Baer’s Law was formerly known as the
theory of recapitulation, in the belief, now discarded, that development
directly represents adults at successive stages in the history of the race.
They may, however, be inferred from it.

(4) The facts of the distribution of animals are yet

another support for the theory of evolution.
Zoogeography. -g foun^ that the animal populations or

faunas of various parts of the world differ, even when
they inhabit regions in which the conditions of life are so
similar that animals native in one will flourish il they be
introduced into the other ; and that the difference between
local faunas increases with their inaccessibility from one
another and with the length of time during which the
regions have been separated. For instance, between
the fauna of Great Britain and that of the adjoining parts
of Europe the differences, though they exist, are very
slight, but distant New Zealand, though British animals
and plants will flourish there, has a very different fauna ;
it has no snakes, only two bats and a frog represent

EVOLUTION

693

mammals and amphibia, it has a number of peculiar
flightless birds (Moa, Kiwi, Ground Parrot, etc.) of various
families, and in fishes, insects, molluscs, worms, etc., it
lacks groups that are common in Britain and possesses
others that are not represented here. Again, while the
fauna of Borneo resembles closely that of the adjacent parts
of Asia, 350 miles away, Madagascar, an older island, has a
fauna which differs greatly from that of Africa, only 250
miles distant. It lacks the great mammals of Africa—
apes, lions, zebras, rhinoceroses, elephants, giraffes, etc. —
and most rodents, and has many peculiar lemurs, reptiles,
and birds. Only by evolution can this connection between

Fig. 520. — The gradual transition between Paludina neumayri
(a), ihe oldest form, and Paludina hoernesi (f). — From
Neumavr.

isolation and peculiarity of fauna be explained. It must
be due to the fauna of each district having had an evolu-
tionary history different from those of the others. The
chief cause of this difference is held to be that the faunas
have lived under different conditions. However alike the
physical conditions may be in two districts, they are
never identical, and often they have been more dissimilar
in past times ; moreover, unless the faunas and floras ol
two districts were at one time identical, which will seldom
have happened, each member of either fauna will also
have an organic environment — enemies, food organisms,
and competitors — peculiar to its district. The longer the
separation has lasted, the greater will be the evolutionary

694 MANUAL OF ELEMENTARY ZOOLOGY

divergence of the faunas. The more inaccessible the
districts have been from one another the more opportunity
there has been for any organism that may have reached
one from the other, or both from a third, to be modified
on the way by evolution.

(5) The facts of pal&ozoology (or the geological history
Paisozooiogy. of animals) are also m favour of evolution.

It is clear that this is the onlv direction in which
we could look for a complete proof of the theory, since all

f 1G. 521. — The bones of the forefoot of a horse compared with those of
earlier members of its family. — From Swinnerton.

(a) Eohippus (Hyrac other turn) (Lower Eocene); ( b ) Orokippus (Middle Eocene) -
c) Mesohippus (Lower Olierocene) ; (d) Hypolripp.us (Lower Pliocene) ; (e) Eauus
(Upper Pliocene to Present). "

the other evidence does no more than enable us to infer
past history from present facts. Unfortunately, owing to
what is known as the imperfection of the geological record ,
such complete proof is impossible. The unsuitability
of the bodies of many animals for preservation, owing
to the absence of hard parts, the destruction, during
the history of the earth, of immense layers of rocks, and
the small proportion of those which remain that can be
examined, bring this about. But it is established that

E VOL UTION

695

throughout geological history, there is a continual change
in the types ot animal lite, leading up to those that exist
at the present day, and in a few rare cases, particularly
among Mollusca (Fig. 520), it is possible to trace fully the
evolution of species, while in others, such as those of horses
(Fig. 521) and ot elephants, the origin of higher groups
can be iollowed in the appearance ot successive genera.

The horses are a classical case of the tracing- of the evolution of a
group of animals by means ot fossils. Starting in Eocene times with
Hyr acotherium ( Eohippus ) of the size and proportions of a fox terrier,
with four toes on the fore feet and three on the hind, and knobbed
(bunodont) teeth, there is known, from successive strata a series of
creatures which became larger and, owing to the lengthening of the
feet, relatively taller, while their side toes dwindled to splints and the
knobs on their teeth became grinding ridges with cement between
them. Finally, in the Pleistocene, modern horses appeared. The
fossils are the remains, not of actual forbears and descendants,
but of animals whose features lead to the conclusion that each is a
little off this direct line ot descent. This evolution went on during,
and was no doubt brought about by, a change in foothold and in food,
due to upheaval ot the land, in which marshy and forest conditions
with succulent vegetation gave place to plains with firmer surface
and harsher herbage.

Even where, as is generally the case, great gaps exist in
our knowledge ol the history of groups of animals, there
are sometimes known fossils which enable us in theory to-
bridge the gull. Such a case is that ol Archceopteryx
(p. 516), pointing backwards from birds to reptiles. We
have traced in Chapter XXII. the history of the gradual
adaptation of vertebrate animals to life on land.

Though zoologists have long been satisfied that evolu-
tion takes place, there has been, and still is.
Evolution °f mu°h conflict of opinion as to how it is brought

Heredity. about. To solve this problem it is necessary

to know (1) how heritable differences between
related organisms arise, and (2) how such differences in-
crease, so that species and higher groups come into being.
To answer the first of these questions, we must now take
note of certain principles of heredity known as Mendelism
after their discoverer, Gregor Mendel, Abbot of Briinn,
who published them in 1866.

We have already seen (p. 598) that, in many cases, when
two individuals between which there is a heritable difference

696

MANUAL OF ELEMENTARY ZOOLOGY

Gregor Mendel, Abbot of lirnnn.

Born 1822, died i£8a

[Fig. 522.]

EVOLUTION

697

in any respect breed together, their peculiarities in this
respect do not blend, but the peculiarity of one parent is
“dominant” in the offspring, while that of the other is
latent or “ recessive.” For instance, the offspring ot a
wild grey rabbit and one ot the black variety are all grey.
Thus the dominant variety remains distinct and is not
weakened by crossing. If/ however, the offspring of such
a cross be bred together, it will be found that the recessive
variety has also not been extinguished, lor on the aveiage
a quarter of the generation will throw back to the recessive
grandparent (in this case, the black rabbit). Further,
when the peculiarities of parents in respect of any feature
do blend in the first generation, it is found that, of a second
generation produced by inbreeding between the brothers
and sisters, only halt the members exhibit the blended
feature, while a quarter throw back pure to each ot the
grandparents. Thus it comes about that both varieties
remain unimpaired within the species. This separating
out of a pair of contrasted characters is known as
segregation .

In the following genealogical table G represents the wild (pure
or “ homozygote ”) grey rabbit, B the black, and (G) the cross-bred
(“heterozygote”) grey, and, in order to represent the results of
various kinds of matings, it is supposed that in the second generation
a brother and sister breed together, and in the third there is one
union of brother and sister and two with homozygote members of
other stocks.

GxB

(G) (G)x(G) (G)

GxG

(G) x (G)

F,

F„

<A

G (G) (G) B

B X B

| „

I I I I

B B B B F3

If the case had been one in which the tendencies inherited from
the two parents blended their effects, instead ot either ot them domin-
ating the other, it could be represented by the same table, but then
(G) would stand for an individual of a colour which arose from
the blending of those of the parents. Fig. 523 shows such a case.

69 8

MANUAL OF ELEMENTARY ZOOLOGY

These facts are explained as follows. Each germ, as
Mendeiism. we have seen, contains in its set of chromo-
somes all the factors 1 that are necessary to
bring about the development of an individual of its species.
These factors are known as genes , and the presence of

^1G- 523-”To illustrate the results of crossing two pure-bred (homo-
E gote) races of Andalusian fowls. — From Morgan.

/>!, the parents one splashed-white and the other black ; F,, the first hybrid eenera-

thlldlV?dUalS °f.)vhich are alike> of a bluish-black shade due to the blend-
ng of the colours of the parents; F2, the second hybrid generation, bred by

a if? individuals together. One quarter resembles each of the parent*
and the remaining half are like Fx. parents

In making this statement we ignore for the sake of simplicity
the facts that the action of the factors in the chromosomes depend
upon the qualities of the cytoplasm, and that a few characters appea
to be inherited through factors in the latter.

EVOLUTION

699

each of them is known by the appearance in the individual
of some character which it evokes. Thus the grey and
black colours of rabbits are due to a pair of genes (or, as-
it is sometimes said, two forms ot a gene).1 for each
character, syngamy has given the individual two genes. It
they were alike the zygote they formed is called a homo-
zygote and develops the character they both bore. It they
were unlike the zvgote is called a heterozygote and one-
gene usually dominates over the other in the development.
At the reduction division when the zygote forms gametes,
the genes again separate, because, it is held, the two-
members of each of the pairs ot genes which are alike in
the homozygote and unlike in the heterozygote are carried
one member in each ot a pair of homologous chromosomes
and these separate at the reduction division to go into-
different germs. In regard to the character in question,
the gametes of a homozygote will all be alike, those ot a
heterozygote will be unlike, consisting ot equal numbers-
of both kinds. When heterozygotes breed together, on
the average half the offspring will be formed by the union
of unlike germs — that is, will be heterozygotes like their
parents. Half the remainder will be homozygotes formed,
by the union ot gametes bearing the dominant gene ;
half will be homozvgotes formed by the union ot those
bearing the recessive gene. All the dominant homozygotes-
and all the heterozygotes will develop the dominant
character. All the recessive homozygotes will develop
the recessive character. Thus three-quarters ot the off-
spring will have the dominant character, one-quarter the-
recessive character. The recessives, it bred with recessives,.
will all breed true. Ot the seeming dominants only one-
third — those which are pure dominants — -will breed true *
the rest — the heterozygotes — will on inbreeding continue-
to throw one-quarter of pure recessives.

The members of a pair of characters which behave in a Mendelian-
way on crossing are known as allelomorphs . Parents may hitter in

1 The term gene is rather loosely used, to mean either an entity
which exists (in one form or another) at the same locus in the same
member of every chromosome set of a species, or the particular case
of such an entitv which exists at the locus in question in an individual
chromosome.

7oo

MANUAL OF ELEMENTARY ZOOLOGY

respect ot a number of such pairs, each inherited separately. The
constitution of their offspring (that is, the average constitution of the
offspring of a number of similar matings) can then be calculated
from the facts we have learnt. Thus when parents are crossed which
differ in two pairs of allelomorphs the hybrids of the first genera-
tion (Fx) will all be alike, exhibiting the dominant character of each
of the two pairs, while the generation (F2) produced by mating such
hybrids consists on the average of nine which show both dominants
to three which show one dominant and one recessive, three which
show the other dominant and the other recessive, and one which
shows both recessives. We may represent these results as follows :
writing A and B for the genes which are dominant and a and b for
their recessives,

Gametes ABXa^ give the heterozygote AB<^

The gametes of the heterozygote wall be, in equal numbers,

AB, A b, tfB, and ab.

The zygotes from such gametes should be in equal numbers of the
following sixteen combinations :

ABAB ABA b

AMB AbAb

aBAB aBAb

abA B abAb

ABtfB AB ab

AbaB Abab

aBaB a Bab

abaB abab

It will be seen that nine of these contain both dominants, three
have each of the dominants alone, and one has neither.

9 cf

is.

i\

Fig. 524. — The chromosomes of the body cells of Fruit Flies
{Drosophila). — After Morgan and others. Flere there are only
eight chromosomes, and the members of each of the pairs which
separate at the reduction division can be recognised by their
shapes and sizes.

$, 1 he chromosomes of the female; T, those of the male; Ab x-chromosomes •
Y, a chromosome which forms a pair with the single x-chromosome of the male’
but is distinguishable from the latter by having a hooked end.

One more case must be considered. In a cross between parents
of which one is a heterozygote in respect of a given character and

E VOL UTION

701

the other a recessive homozygote for the same character, the off-
spring will be divided equally between the dominant and recessive
allelomorphs, as the following diagram shows :

Parents Aa aa

Gametes

Offspring (F,)

A a a

*

\

aa

That is, two kinds of individuals, breeding together, produce, on the
average, equal numbers of each. Now this is what
M^ndelHan usually happens in respect of sex, and the inference that
Character. sex 1S a Mendelian character has been justified by re-
search, though the case proves not to be so simple
as that which has just been stated. The probable explanation of
the facts is as follows. One sex is a nomozygote and the other
a heterozygote in respect of a factor which in double dose (the
homozygote) confers sex of one kind, but in single dose (the hetero-
zygote) is ineffective ; and in the latter case the individual is of
the opposite sex.1 The heterozygous sex is sometimes (Birds, Lepi-
doptera) the female, sometimes (other insects, mammals) the male.
The chromosomes which carry the sex-factor in question (x-chromo-
somes) can often be distinguished, and then it is found that two are
of that kind in the cells of the homozygote sex and only one in those
of the heterozygous, and that accordingly the gametes of the homo-
zygote have each an x-chromosome, while only half of those of the
heterozygote possess one. If a be taken to represent an x-chromo-
some, and A the absence of one, the above diagram represents these
facts.

From considerations into which we cannot enter here, it appears
that the genes have each a definite position in a chromo-
Senes- some and are there arranged in linear order, the genes

for allelomorphs standing in the same position in homologous chromo-
somes. Characters whose genes are in the same chromosome will
usually be inherited together — thus in Drosophila black colour and
curved wings are generally found in the same individual. This
connection is known as linkage. Owing to crossing over it is not
invariable. What the nature of genes may be is unknown, but it is
clear that their action is dependent upon circumstances.- The pre-
sence of one of them is often betrayed by the existence of a particular
character in the organism, but its production of this character is
dependent (a) upon external conditions — for instance, pigmentation
is in some cases only effected by the gene concerned if the temperature

1 By the action, probably, of an independent factor, which is
present in all individuals, but is countervailed by the double dose of
the sex-factor of the homozygote just mentioned.

702 MANUAL OF ELEMENTARY ZOOLOGY

be favourable — (b) upon interaction with other genes. The genes
in any organism make up a getie complex in which they react with
one another to produce the characters of the organism. The effect
of each gene depends upon the presence of several others, and each
gent affects other processes than the production of the conspicuous
character (it any) by which its presence is recognised. Thus a gene
which gives white eyes in Drosophila also changes the colours of the
itestis sheath and the shape of the spermatheca.

The mechanism of heredity being what it is, heritable
'Mutation. differences between related organisms can only
arise by the origin of new genes or the re-
combina on ot existing ones. The origin of new genes
is known as mutation. It is due to a change occurring
m a chromosome at the site of a pre-existing gene. For
instance, in a race of water fleas which had been found
to thrive best at a temperature of 20° C. and to die at
26 C., there suddenly arose by a change of this kind a
strain which flourished best at 270 C. and died if the
temperature tell to 20 C., in the same way, there appeared
in the Fruit Fly Drosophila melanogaster individuals of a
black . colour instead of the ordinary grey. Though
mutations have chiefly been studied in laboratories it is
known that they occur in wild nature. Mutations may
have obvious and considerable effects, or be so small as
to be detectable only by close examination. At their
first appearance they are usually recessive and also
deleterious, by disturbing the gene-complex and so
weakening the constitution and sometimes by altering
the relation between the animal and its environment—
making it, tor instance, conspicuous, or weakening its
defence. It seems, however, that an unfavourable
mutation usually becomes recessive and a favourable
one dominant. Probably this is due to the effect on the
gene-complex of selection (see below). The larger the
mutation the more likely it is to be harmful. It is believed
that evolution usually takes place by the accumulation of
small, favourable mutations. How mutations arise is not
known. It has been supposed that alterations brought
about in the body of an individual by circumstances
<“ acquired characters ”) affect the germ cells so as to
be inherited — -that is, cause mutation — but this, as we
shall see, has never been proved to occur. It has been

E VOL UTION

703

supposed that the germ cells have an inherent tendency
to mutate (and that, according to some authorities,
•continuously in certain directions) but this also has not
been proved. Experiments have shown that mutations
may arise in consequence of the influence of external
factors upon the germ cells : thus by the effect of x-rays
heritable modifications have been brought about in fruit
flies ; but there is no evidence that any of the agencies
so used in the laboratory are the normal ones in nature.

Recombination of genes takes place in two ways — by the

Recombination. un]on different sets of chromosomes in sexual
reproduction, and by the exchange of genes
between chromosomes of different sets through crossing over
at meiosis. It does not produce such novelties as mutation
does, but it reassorts them, making new combinations of
•characters — as between colour and length of hair. More-
over, by altering the gene complex it alters the effects
of individual genes, one gene, for instance, increasing
or diminishing the effect of another— as by deepening or
enlarging patches of colour. Any of these effects may be
valuable or detrimental to the animal, and the existence
of the species will accordingly be affected by the persistence
in it of certain combinations of genes. Generally speaking
the plasticity of feature which recombination gives is
valuable in providing an assortment of kinds of individuals
to meet the vicissitudes of the environment.

For the second part of the problem of evolution — how
heritable differences between individuals are
Evolution*- increased, so that species result and diverge
Lamarckism, until genera and higher groups appear—
various solutions have been propounded.
They are of three kinds. One of these supposes that the
modifications which arise in each individual in the course
•of its life, by the action of its surroundings upon it, by
its activity in response to the stimuli it receives from its
surroundings or by the dwindling of certain of its organs
from lack of use, are inherited in some degree by its
offspring, and that the accumulation of such small
modifications produces at length a different kind of
-animal. Thus, since it is known that one effect of cold
upon a mammal is to increase the growth of its hair, the

704 MANUAL OF ELEMENTARY ZOOLOGY

Jean -Baptiste de Lamarck.

Born 1744, died 1S39.

[Fig. 525.]

E VOL UTION

705

long fur of species which live in cold countries might be
supposed to be due to the inherited effect of the climate.
Again, the effect of use upon muscles is to increase their
size, and in this way the great size of the wing muscles
of birds of strong flight might be supposed to have been
brought about in the course of many generations. On
the other hand, the dwindling which is undoubtedly
caused in the organs of individuals by disuse might in
time bring about permanent degeneration, such, for
instance, as that which is found in the eyes of animals
which live in dark caves. This hypothesis is known as
the Lamarckian theory , from the name of its greatest
exponent. It is rejected by most zoologists at the present
day on account of the lack of satisfactory evidence that
modifications which are produced in the course of the
life of an individual are transmitted by it to its offspring.
Such modifications are known as acquired characters and
over the question of their inheritance discussion is still rife.
The obstacles to belief in their transmissibilitv lie not only
in the fact that numerous experiments and observations
have hitherto failed to prove such transmission but also
in the difficulty of conceiving any way in which modi-
fications in other parts of the body can so affect the
germ-cells as to be handed on to the offspring.

Many experiments have been made upon this point. In most of
them the “ acquired ” character, whether produced crudely, as by
cutting off the tails of mice, or more naturally, as the effects of altered
nutrition upon the shells of water-fleas, is not inherited. There are
some of which the result has been claimed to prove the inheritance
of such a character, but all these are, for one reason or another,
inconclusive.

Darwinism.

Another hypothesis as to the way in which evolution is
brought about is the theory of natural selection ,
known as Darwinism after the great naturalist
by whom it was formulated. It supposes that the trans-
formation of species is caused by a greater destruction by
adverse circumstances of certain kinds of individuals in
each generation before they can breed.1 The result of

1 Of course the eliminated individuals would not always be killed
before they could breed. In some cases the reproductive period would
merely be cut short so that the number of offspring was lessened.

45

Fig. 526. — British breeds of sheep.

Showing various races established by breeding from selected individuals,

706

E VOL UTION

707

this will be that the peculiar features of these individuals
are inherited by fewer of the next generation and therefore
gradually cease to appear in the species, or in that group
of its members which has been subjected to the circum-
stances in question. Thus in a cold country those members
of a species of mammals which had not thick fur would
be more liable to die of cold or be so enfeebled that they
could not compete with the rest for food or mates, or
in a herd of wild horses pursued by wolves the slowest
would be killed, so that the next generation would be
descended from those members of the species which were
best clad or swiftest, as the case might be, and would of
course inherit their peculiarities. The result of this process
would be the selection by Nature of certain individuals
to breed, just as a breeder selects sheep with thick wool
or without horns, or rabbits with long ears or fur, and,
breeding from these in preference to the rest, alters his
breed of sheep or rabbits. (See Figs. 526 and 393.)

Evolution by natural selection depends upon three
factors : variation, the struggle for existence,
Factors in and heredity. (1) We have seen that all

Evolution. animals are variable. It is true that variations

which are produced during the lifetime of the
individual by the action of the environment upon it or
by its use or disuse of organs are probably not transmitted
by it, but, as we have seen, there are other characters
which are its birthright, being due to the make-up of the
fertilised ovum which produced it, and these are inherited
by some or all of its offspring, so that selection of them
will affect the next generation. Some species are more
variable than others and will therefore offer more scope
for selection. Species often appear unable to vary in
certain directions — thus roses are never blue — but apart
from this variation is at random ; for evolution to take
place direction has to be imposed upon it by the sifting
action of selection. It is often urged that variations are
too small to give an effective advantage in the struggle lor
existence, but on the one hand they may be of considerable
magnitude, and on the other it has been shown that quite
a small advantage is in the long run effective (p. 709).
(2) The struggle for existence involves, not merely reaching

7o8

MAX CAL OF ELEMENTARY ZOOLOGY

a certain standard of fitness to cope with the surroundings,
but a. competition between individuals, because, while the
offspring are always more numerous than the parents, the
total number of individuals in the species does not as a
rule increase, being already as many as the conditions of
food, enemies, etc., will allow. A pair of robins will
produce ten or more young in a year, yet, since the number
of robins does not increase, only two of these can survive.
This is an exceptionally small death-rate. Manv animals
produce thousands of offspring : the blow-fly, for instance,
gives rise to a progeny of 20,000. Now it is impossible
to believe that the destruction which this involves is
altogether haphazard. Some of the individuals will be
feebler, or slower, or less cunning, or less protectivelv
coloured, or less warmly clad than the rest, and it is
certain that these will as a rule be the first to be destroyed,
and that the survivors will generally be above the average
of the previous generation in regard to the characters in

which selection has taken place.

©

The objection that much destruction is indiscriminate, as when
spawn is eaten by fishes or minute sea animals by whales, is nor
valid, since what remains is sufficient in quantity and contains a
tair sample ot all the varieties for selection.

It should be noted that selection is not only an innovating but also
a conservative force. It tends to destroy all new characters that are
detrimental to the relation of the species with its environment. Thus
it maintains existing species as well as, upon occasion, moulding
new ones. It is the lack of selection maintenance that, by non-
suppression of degenerative effects of mutation, brings about the
degeneration of useless organs, which is sometimes regarded as a
difficulty for the theory of natural selection.

(3) The alteration which is thus made in the average

characters of the species will be maintained by the action

off heredity. It has been alleged as an objection to the

theory of evolution by natural selection that any large
variation in a favourable direction, though it may lead
to the survival of the individual in which it occurs, will
nearly always be weakened in the next generation by
what is known as the “ swamping effect of intercrossing.’3 * * * 7

That is to say, the exceptional individual will probably
mate with an average member of the species and their
offspring will be intermediate between them, and thus in a

EVOLUTION

709

few generations the favourable variation will have become
so slight as to give no effective advantage in the struggle
for existence. This difficulty, however, disappears when
it is recognised that characters due to genes may temporarily
vanish if the gene be recessive, but are not abolished or
weakened.

Another objection which is dispelled by a knowledge of Mendelism
is that the numerous variations which are necessary to constitute
•even a fairly simple organ are unlikely to occur together. Genes
■can persist in a recessive condition till there occur others in combina-
tion with which they give at least a minimally effective rudiment.

Natural
Selection
at Work.

Natural selection has been observed actually in process
in certain cases. Thus certain of the insects
known as water boatmen (Corixidae), which
resemble in their colouring the background of
the pond in which they live and are thus
rendered inconspicuous, have been found to be selectively
•captured for food by fishes, which take a heavier toll of
those that are least like their backgrounds. Again, it was
•observed in America that after a severe storm sparrows
whose wings were unusually large or small had been
killed in greater numbers than the others. Calculations
have been made of the rate at which such selection would
.alter a species. Obviously this depends upon the number
of individuals in which the new character appears and
upon its effectiveness in the struggle for existence. It has
been shown that a mutation which was a Mendelian
•dominant if it appeared in 1 per cent, of the members of
a species and gave only a 1 per cent, advantage to its
possessor, would after 500 generations be present in half
the population, and after 1658 generations be possessed
by 99 per cent.

In the course of our discussion of natural selection we
.... . have dealt with various objections which have

•Orthogenesis. . . J

been raised against that theory. I here remains

■one of considerable importance. Palaeozoology reveals
the fact that not only has evolution in many groups of
animals proceeded in the same direction for long periods
•of time (as, for example, it did in horses), but in some
such cases its course appears to have been of no advantage
to the animals and eventually to have become dis-

MANUAL OF ELEMENTARY ZOOLOGY

710

advantageous, thus, for instance, the extinct group of
mammals known as Titanotheres , as their size increased,
evolved disproportionately large and, it is alleged, un-
wieldy horns ; and in the bivalve mollusc Gryphcea the
shell curved to one side till it could not open properly. In
such cases it is argued, selection cannot have been the
directing factor, and it is therefore supposed that mutation
in a definite direction was caused by some force in the
organism. From this some authorities have proceeded
to suppose that a similar force has played a major part
in all evolution, with selection as a merely subsidiary
limiting factor. Evolution of the kind alleged by this
theory is known as Orthogenesis. Most zoologists, how-
ever, are unwilling to admit the existence of any such
mysterious directive force as it involves. Nearly all the
facts upon which the theory is based are susceptible of
explanations compatible with natural selection (as by the
connection of useless with useful characters, by “ sexual
selection,” and so forth) and it is held that similar ex-
planations will be found for the residue.

Selection

Overrides.

As against either of the theories that suppose evolution to take place
by directed mutation— Lamarckism or Orthogenesis — -
the theory of Natural Selection from among random
variations has the advantage that selection would un-
doubtedly override any directed mutation that can reasonably be
supposed to occur. The intrinsic mutation upon whose direction
orthogenesis is held to depend takes place at a rate which is
insufficient to override selection. It has been shown that a reduction
in viability of one-tenth of one per cent, as the result of a mutation
would result in adverse selection which would override mutation at
the highest rate ever observed in nature ; a mutation which increased
viability by o-i per cent, would be spread by selection. The heritable
changes upon which Lamarckism depends would certainly be similarly
overriden, since they must be so small as hitherto to have escaped
detection.

CHAPTER XXX

THE ANIMAL IN THE WORLD

Our survey of zoology is drawing to a close. We
began by observing that an animal is an
objects^!?8 organism and, in general terms, how and
Nature. to what end its organisation functions.

We went on to examine a series of animal
organisms and to investigate the modes in which they
have come into being both as individuals and as kinds
of individuals. It remains for us to survey briefly the
relations in which such beings stand to other material
things.

The outstanding fact about an animal, as about a plant.

The Environ *S *ts re^at^on to surroundings, or, as thev
mentEnv,r0n' are collectively called, its environment. We

have seen that its life consists in adjustment to
these, in avoiding the dangers and taking advantage of
the opportunities which they present. Four principal
factors are concerned in this relation — the ground or
substratum , if any, upon which the organism stands, the
medium (water or air) which bathes it, the heat and light
which it receives from or can lose to its surroundings,
and other organisms. Of these factors, the substratum
has in most cases comparatively little importance, and
we shall not consider it farther.

The medium, on the contrary, is very important. It
exerts pressure all over the organism, supports
Medium. jb may transport it from place to place, affects
its movements, and controls all exchange,
whether of matter or of energy, between it and the world
around it. The medium of the first organisms must have
been water : in dry surroundings protoplasm could never

712

MANUAL OF ELEMENTAL V ZOOLOGY

have originated. Water is a tactor of the first importance
to living beings. Within the organism, it is essential as a
constituent of protoplasm, and in higher organisms as
a means of internal transport. When it is the medium,
it has both advantages and dangers. On the one hand, it
is then at hand in plenty, may carry food and other needful
substances, inorganic or organic, in solution or suspension,
and affords support. On the other hand, according as the
concentration of salts in the watery medium be greater or
less than that within the organism, water or dissolved
substances will tend to pass by osmosis to or from the
body, m which their proportions may thus be dangerously
altered. It is for this reason, for instance, that marine
organisms usually cannot live in fresh waters, and vice
versa. We have noted, in several of the animals that we
have studied, how when dangers of this kind exist for an
animal in its normal environment protection against
them is provided by lowered permeability of the surface
of the body and by the excretion of water or solids accord-

ing as one or the other threatens to be in excess. When
the medium is aii, the animal organism encounters a
different set of difficulties. It spends more energy to
obtain water and is liable to lose water by evaporation
and hence, save in very moist places, needs an even less
permeable covering than fresh-water creatures. It can
larely obtain food from the medium, cannot entrust its
gametes to the latter and must, therefore, have internal
fertilisation, cannot stand up above the substratum without
a strong skeleton, must have respiratory organs adapted
to the use of the more mobile medium, and. as we have
seen (p. 80), has to adopt various chemical expedients to
avoid being poisoned by its nitrogenous excreta.

I he relations between an animal and other organisms
are, of course, not only relations with other
between 8 animals. 1 here are included among organisms
Organisms. many other creatures, of which the best known
are the plants, though some cannot rightly be
said to belong either to the animal or to the vegetable
kingdom.. We have already observed the differences be-
tween. animals and plants and noted that their modes of
nutrition differ and are complementary.

THE ANIMAL IN THE WORLD

713

Drganisms
intermediate
between
Animals and
Plants.

The fact must not be overlooked that intermediate organisms are
known. These are not such creatures as polyps, which
are true animals with the habit of remaining fixed in one
spot, or the fungi, which are probably true plants that
have lost their chlorophyll and consequently modified
their mode of nutrition, so that they require somewhat
more complex food materials. The real intermediate organisms are
such creatures as Chlamydomonas and Polytoma , which have the
power of locomotion like animals, but possess like plants either
•chlorophyll or starch or both. They are classed by zoologists as
flagellate Protozoa (Mastigophora) and by botanists as Algae. Their
-existence is a reminder that, fundamentally, all living beings belong to
the same stock.

In this connection must be mentioned the Bacteria , which are
minute, rod-like or spherical organisms often classified with the plants,
but of which it is perhaps more true to say that they are neither plants
nor animals, but a third kind of living beings.

Here must also be mentioned some special kinds of metabolism
which differ greatly, though not in principle, from those
^P*®1’** of ordinary plants and animals. We have already seen

file a 0 isms. ^ various organisms, of which Ascaris

and other internal parasites are examples among animals, and the
yeast fungus and many bacteria among plants, are able to live without

free oxygen. Such organisms obtain
their energy by the decomposition of
oxygen-bearing molecules of organic
substances. Thus the minute fungi
of which yeast consists decompose
grape sugar into alcohol and carbon
dioxide with the evolution of so much
energy in the form of heat that the
temperature of the surrounding solu-
tion rises. This is the process of
fermentation employed in the manu-
facture of alcoholic liquors, and from
it the term fermentation has been
applied to other processes, such as
the souring of milk or wine, and the
putrefaction of dead bodies by
bacteria (p. 72,5), in which an organ-
ism brings about a change in a mass
of matter which is very great com-
pared with that of its own body. It
has been shown that in many such
cases there is formed by the organism
an enzyme (p. which brings about the fermentation without itself
being destroyed in the reaction. Such an enzyme has been found in
the protoplasm of yeast. Quite a different class of fermentation is
found in certain bacteria which obtain energy by the oxidation of
inorganic substances, such as sulphuretted hydrogen, sulphur,
ferrous salts, and ammonia. The energy is used in building up

\ b
\

<v

v*

//'*>! fi
O'" a

■a

Fig. 527.— Bacteria of
decay.

Bacillus vulgaris, the principal
bacterium of decayin'? flesh ;

b, Bacillus subfolis. very com-
mon in decaying plant tissues;

c, Bacillus coli, the commonest
bacterium of dung.

714

MANUAL OF ELEMENTARY ZOOLOGY

organic from inorganic substances (ckemosyntkesis), as the energy of
the sun s rays is used by green plants in photosynthesis. It is by an
oxidation of this class that ammonia compounds derived from" the
oodles of organisms are converted into nitrates and thus made
available for the higher plants (p. 30).

The fundamental difference in nutrition between animals

the Balance an<^ P*ants Pas> as we saw> very important
of Nature. * consequences in their relation with the rest

of nature and with one another. In their
action upon the inorganic world these two kinds of
organisms bring about precisely opposite changes, and do
so in such a way that

Organic substances

each sets up conditions
favour able to the
activity of the other.

Plants provide food and
oxygen for animals;
while animals, destroy-
ing this food, provide
for the use of plants
carbon dioxide andT, „ .

simple nitrogen X“gh & td/efo"

pounds (though the organisms.

Utter are usually not

available for the use ot plants till they have been altered
by the action of bacteria in the way mentioned above).
The result is a, circulation of carbon and of nitrogen
through the bodies of organisms. It will be seen that this

Fig. 529.— Nitrosomonas , ?l bacterium which performs the first stage
of the oxidation of ammonia to nitric acid.

circulation of matter is accompanied by a transference of
energy. The whole of the energy of the life both of plants
and ot animals is derived in the long-run from the energy
of the sun s lays stored by plants in the complex substancef

THE ANIMAL IN THE WORLD

7i5

they manufacture. It is stored by plants : most of it is
not set free till it reaches the bodies of animals. There
is, of course, no circulation of energy. That which is set
free from the bodies of organisms is lost to them, and
has to be replaced by the fixing of more energy from the
sun’s rays by plants when they work up the excreta of
animals. The whole animal kingdom may be regarded as
a vast complex system which, by means infinitely more
subtle than those which are possible in the inorganic realm,
disposes of the material and energy accumulated by plants.

Rays of the sun absorbed by plants.

Stored energy of plant substances.

Energy freed in
the life of plants.

Stored energy of
animal substances.

Relations
between
Animals
based upon
Nutrition.

Energy freed in the life of animals.

Fig. 530. — A diagram of the energy of organisms.

The relations between the different kinds of animals are
almost all based in the long-run upon nutrition.
Only between members of opposite sexes of
the same species are there relations of another
kind, namely those which are based upon
reproduction. For the rest, either animals
compete for the common supply of food which is derived
directly or indirectly from plants, or some of them serve
others for food, or in rarer cases they assist one another
in the quest for food or in defence against enemies which
would use them for food. We may class animals accord-
ing to their food as omnivorous, herbivorous, and carni-
vorous, or according to their method of obtaining it as
free-living, parasitic, symbiotic, and commensal. Most of
these classes need no further comment. We have seen

7 1 6

MANUAL OF ELEMENTAL V ZOOLOGY

instances ot the ways in which omnivorous, herbivorous,
.and carnivorous animals are nourished. We have dealt
with parasitism and symbiosis. Under the head of
commensalism a large number ot curious instances of
co-operation between animals is known. One of these
must suffice here.

The hermit crabs
are crustaceans re-
lated to the crayfish,
with the abdomen
soft, owing to the
thinness of its cuticle,
and twisted so that
it will fit into
the empty shell of
molluscs like the
whelk. They search
for such shells, often
fighting one another
for possession, so
that they are some-
times called soldier
•crabs, and they
anchor themselves
into their shells by
means of the limbs
of the sixth ab-
dominal segment.

W hen they are
attacked they with-
draw into the shell
by the contraction
ot a muscle in the
abdomen, but often
this does not save

Ug. 531.— A hermit crab withdrawn from
its shell. The anterior legs are cut
short. — From Thomson.

hd.> Head ; th , thorax ; abd. , abdomen.

them from being eaten by fishes, of which they are a very
favourite food. They are very active, and are constantly
travelling in search ot lood, dragging about their shells
with them. Sea-anemones are, as we have seen, polyps
related to Hydra , but more complicated in their internal
structure. Owing to their nematocysts they are distasteful

THE ANIMAL IN THE WORLD

7i 7

to fishes, as is shown by the fact that when alive they will
not serve, for bait. They cannot pursue food, but must
wait till it comes within reach of their tentacles. Now
certain kinds of sea-anemones are found on the shells
of hermit crabs. Here they are never molested by the
owner of the shell, and benefit by the constant change of
feeding ground and by fragments of food which are let
fall by the hermit crab, to obtain which some of them
stand with the mouth on the lower side of the shell, which
their base enwraps. In return the crab obtains protection

Fig. 532. — Sea-anemones on the shell of a hermit crab. — After Andres.

from fish, which are kept from eating it by the stinging
powers of the anemone. Within the shell there is often
found a species of Nereis, which with its horny jaws steals
the food out of the very pincers of the crab, without, so far
as can be seen, conferring any benefit in return, and must
therefore be regarded as a parasite.

Upon the exploitation by animals of the means of
living that the world affords, two further com-
Radfation. ments may be made. First, that the more
highly organised do not always oust the lower.
A?noeba and Man still exist upon the same planet, though
they make very different jobs of life. Secondly, we may

flG. 533. Members of the pelagic fauna, considerably magnified,

ThC became ^noJ^C’fl0ng t0-the plankton 5>r dating fauna, so called
, us,® “f Powers of swimming are insignificant compared with the
eoSth of the currents of the sea. They derive their food from minute

t'TOtleilv’/rV11 tUrn Serne *eed larger pelagic animals, such as
A7tn>e7breat JePy'nsh, herrings, and whales.

1, NocHluca , a large, phosphorescent flagellate : 2, medusa of a hvdroid • s a

wo™ mSSJr- TfV ,«■ • rTtW ‘° Cyc^s WSJ Arrow

on,Q1 , 3, 4, 7, larvas of a sea-urchm, a worm related to Nereis

^disTrihJfterf^ rf ate<11 t0u the lobsters- Many other bottom-living forms
are distributed m larval phases such as these (see p 38-;) °

718 h

THE ANIMAL IN THE WORLD

719

note that often, whether by increased complexity or
merely by diversity of organisation, there are from a given
situation more ways of advance than one. This leads
to what is known as adaptive radiation. The classification
of any group of animals, which in another aspect is its
genealogical tree, is an expression of this. Among
mammals, for instance, there are tree-climbing, forest
and marsh, and plain dwelling, burrowing, aquatic, and
flying groups, and in most of these we find carnivorous
and herbivorous members. In each such mode of life
the species that adopt it come into relations, as devourers,
or competitors, or prey, with members of other groups —
reptiles, birds, fishes, insects — which have taken the same
course.

The assemblage of animals which dwells in any given
Faunas locality or kind of locality — its fauna — is, as

will be gathered from the foregoing pages, no
collection of independent units, but a complex system of
beings in constant interaction with one another and
with their surroundings, both living and lifeless. We may
recognise geographical faunas , which belong to localities,
and oecological faunas , which belong to kinds of locality.
These, of course, are cross divisions, for most local areas
have parts of different kinds — as an island may have
mountains, plains, and streams — and most kinds of local
conditions turn up again and again in different places —
as, for instance, do those of fresh-water ponds. The kinds
of animals which make up a geographical fauna are
determined by the past history of the locality (see p. 692)
— as, for instance, by causeways of land which in earlier
ages allowed the immigration of certain animals but
disappeared before others were evolved — and such a fauna
has no features which are common to all its members
except in so far as it may happen to be also an cecological
fauna, which it will be when the locality is throughout of
one type. But an oecological fauna has usually well-
marked common features which appear in different forms
in all its members, however unrelated they be in evolution,
and enable them to live in the conditions which are common
to them all. Thus, we may divide oecological faunas into
those of the land and the water , and it then appears that land

720

MANUAL OF ELEMENTARY ZOOLOGY

animals have means of breathing in air, internal fertilization,
a covering which hinders the loss of water from the body, an
absence of special swimming organs, and usually strong
skeletons in correspondence with their life in a medium which
does not support them as water would . Differences of colour,
clothing, shape of limbs, etc., distinguish the faunas of
mountains, deserts, snowfields (see Frontispiece ), marshes,
and so forth. Again, the oecological subfaunas of the sea—
the shallow-water fauna , the deep-sea fauna , and the free-
water or pelagic fauna — have each their own character-
istics. The pelagic fauna, for instance, consisting of
animals whose life is passed in independence of any solid
substratum, usually shows in its members great buoyancy,
attained either by extreme delicacy of tissues, as in the
jelly-fish, or by air-bladders and like arrangements. It has
also a curious and unexplained tendency to phosphores-
cence, which at times causes it to light up the surface of
the sea in a well-known and beautiful manner. The deep-
sea fauna, living in darkness or half-darkness, where plant
life is impossible (p. 26), is blind or has very powerful eyes,
and is carnivorous, deriving all its food in the long-run
from the falling bodies of pelagic animals. Its members
also are often phosphorescent. The study of such faunas
and their characteristics is one of the most fascinating
chapters of Natural History.

We have already briefly considered the general relation
in which living beings stand with their lifeless
Lifeless3"11 surroundings. We have seen that the living
Things. being — that is, in the long-run, its protoplasm,

which alone is active in it — delicate and un-

stable though it is, by a purposive reaction maintains its
existence in the face of the forces of inorganic nature.
These forces are very powerful and unceasingly at work, and
would surely destroy the organism but for its activity. Act-
ing from without upon a lifeless object they find in it a toy or
a passive victim ; but in the living being they set in action
a machine of great efficiency which reacts upon them for
its own benefit. In so doing it is engaged in the “ struggle
ior existence.7 The result of this is, in the animal, ability
to exist amid surroundings which would destroy it but for
its life. In the world as a whole the result is a complication

THE ANIMAL IN THE WORLD

721

of the action of its forces which is a factor of enormous
importance in the system of nature. By the living machine
these forces bring about results which they could not
otherwise accomplish. The history of this reaction is
written large in the very substance of the earth. Enormous
beds of chalk and limestone composed of the skeletons of
minute marine animals, countless coral reefs and islands,
vast areas covered with vegetable mould by the action of
plants and earthworms, and great tracts of country whose
face has been changed by human activity, bear witness to
its existence ; and since the coming of Man it has pro-
gressed more and more rapidly till it promises to dominate
every other terrestrial agent of change in nature.

In conducting this struggle the organism turns to its own
use a part of the forces of nature, and that is what is happen-
ing in the circulation of matter and transference of
energy through the bodies of organisms. Only by a
continual change of its substance can the organism keep in
being. It is thus, like a stream or a whirlpool, an object
in nature which remains in existence in spite of a continual
change of its substance ; but whereas the existence of the
stream or the whirlpool is maintained by the action of
external forces, the existence of the organism is mantained
by a reaction in which energy- is liberated and directed
from within. As we saw at the outset of our studies, the
organism is in this respect unique.

The question suggests itself whether this peculiarity of
living beings be due to their possessing any property that
is not found in the rest of nature — whether, that is, life
differs fundamentally from the processes of the lifeless
world, or, on the contrary, could, if we knew enough, be
completely described in terms of those physical and chemi-
cal laws which suffice to describe the events which occur in
inanimate objects. This is the ultimate problem of Biology.

In many respects the events in living beings are cer-
tainly not peculiar to them. Disintegration with evolution
of energy is a common process in lifeless things. In them
it may be started by stimuli which have no relation to its
magnitude, or may be spontaneous. There is nothing
unusual in the appearance of the energy as chemical work ;
and even contraction, secretion, and conduction, though
46

Fig. 534- — Stomias boa , a phosphorescent fish taken at a depth of 1900 metres. Less than half

THE ANIMAL IN THE WORLD

723

they present difficulties, will probably eventually prove
capable of explanation. Absorption is common in lifeless
nature and assimilation has at least suggestive analogies
in what is known as “ autocatalysis.”

But when we regard the direction of living processes we
are on different ground. The gross mechanism of the
organs, and of the nervous and hormone systems by which
they are regulated, is describable in physical and chemical
terms. But the origin of these systems is not. Why does
the solution of colloids and crystalloids which we call
protoplasm, which makes and uses this coarser mechanism,
conduct its traffic with its surroundings in such a way as
to conduce to its survival ? To this question there is as
yet no answer.

Two warnings, however, must be given lest the nature
of these peculiarities be misunderstood. It
tfonUahfiCa" must in the first place be recognised clearly
that they do not dispense the bodily machine
from the obligation to act, like other machines, in con-
formity with the principle of the conservation of energy.
Like a steam-engine, the body can do nothing except in
virtue of the energy which it obtains, in the long run,
from the food which provides its fuel. Secondly, in the
purposive direction of the processes of life we have a
phenomenon which, though it is unique, is not necessarily
incapable of explanation.

So far we have been dealing with matters of fact. The
existence in living beings of the peculiarities
and vital iTm. we have just stated is indisputable. Over
their explanation, however, rages the great
controversy of Biology. The mechanistic school of
biologists regards all the phenomena of life as due to the
laws of physics and chemistry, and looks forward to the day
when the extension of our knowledge will enable us to
explain them all in terms of these sciences. The apparent
exceptions to the laws of chemistry and physics are, for
it, not real exceptions, but are due only to our ignorance
of the details of the processes in which they occur. The
purposive direction of the life processes is due to the
structure of the living machine, though here again our
ignorance does not allow us to see how it is brought about.

724

MANUAL OF ELEMENTARY ZOOLOGY

The vitalistic school believes that the present impossibility
of understanding biological phenomena in the light of
physical and chemical facts is due to the operation in
living beings of a further factor or factors, without a know-
ledge of which life will never be explained. It is held that
no machine can be conceived which would direct its own-
activity in the way in which the activity of a living being
is directed — that, indeed, the word “ machine ” can only be
used in a limited sense of a living organism. As to the
nature of the factor in question, if it exist, nothing is known,,
though there is a tendency among vitalists to regard it as
psychical, but it must in any case be able to direct the
physical and chemical forces of life without increasing or
diminishing their energy.

The problem of the peculiarities of life is bound up
with that of its origin. If we understood
JjjJ origioi how life arose we should know to what its
Bodies. peculiarities are due, though it is true that we
might well know this without being able to
reproduce it. Life, as we have seen, is found only in
organised bodies , though not all organised bodies are alive.
We may class material objects as follows : —

Organised

Unorganised

Living

Dead j T .c ,

j Lifeless.

It is possible that if we knew how organised bodies first
arose we might understand the origin of life, but this is by
no means certain. The organised body is one thing. The
life in it is quite another. On this point cases of suspended
vitality (p. 145) are an interesting commentary.

Organised bodies are characterised by peculiarities of
structure, of composition, and of origin.
Organised" °* fheir peculiarities of structure and composi-
Bodies. tion we have already studied. We may now

consider their origin. Here we are met at
the outset by a difficulty. We are bound to believe that
these complex structures have arisen from matter in its
simpler unorganised form. The theories of special creation
and of evolution agree in regarding the unorganised world
as primary and organised bodies as derived from it. Yet

THE ANIMAL IN THE WORLD

725

organised bodies, alive or lifeless, never, in the present
state of nature, arise from unorganised matter. Every such
body arises by the processs o± fission Irom a previously
existing living body. In the case of the higher organisms
this is no more than a truism. We know that every
individual of the familiar kinds ot animals and plants

had a parent from whose body is has arisen. But there
are cases in which parentage is not so obvious. It the
dead body of any organism be heated strongly, so as to
kill any living things that
may be in it, and placed in
an apparently clean vessel,
closed so as to prevent the
entry of living organisms,
it will nevertheless putrely,
and microscopical examina-
tion will show that putrefac-
tion is accompanied by the
appearance of innumerable
minute “ micro-organisms ”
of various kinds (p. 713),
some of which are indeed
the cause of the putrefac-
tion. Have not these been
developed from the sub-
stance of the dead organism
without the intervention of
life ? The answer to this
question has not been easily
reached. One of the hardest-
fought controversies in the
history of science has been between the supporters on the
one hand of the theory of Abio gene sis or Spontaneous
Generation — the origin of living from lifeless matter -
and on the other of the rival and now victorious theory of
Biogenesis , which maintains the aphorism omne vivum e
vivo. Finally, however, it has been demonstrated that
organic matter will not develop micro-organisms it,
after being properly sterilised by heat, it be placed
while it is still hot into a sterilised vessel to which
only filtered air has access, or if it be sterilised in

Fig. 535-— A hot-air steriliser —
from Muir and Ritchie.

Note the plugs of cotton wool in the
mouths of the vessels.

726

MANUAL OF ELEMENTARY ZOOLOGY

such a vessel.1 The explanation of the appearance
of such organisms in other circumstances is that they
give rise to minute germs or “ spores ” which are
capable of existing in a dried state, and in that state
are carried by the air, to germinate when they fall on
suitable ground ; and that such spores were present
either in the substance which putrefies or, if that has been
rendered sterile, in the vessel which contains it, or in air
which has access to it. The micro-organisms are killed
both in the organic matter and in the vessel by sterilisa-
tion, and their germs are filtered out from the air which
enters as the vessel cools. In this, as in every other
instance which has been carefully investigated, it is
proved to be the case that organised bodies arise only
from living bodies of their own kind. The first organised
bodies must have arisen from unorganised matter, and
it may be that the conditions in which this happened will
some day be discovered and perhaps even reproduced.
But at present it is true that living organised bodies are
necessary for the reproduction of their kind.

It will be seen that there are two distinct wavs in

J

which life is necessary for its own continued
of'ufe.nt,nU,ty occurrence. It alone can provide the kind of

body in which it occurs, and such a body
cannot live unless the life of its parent be continued in it.
Thus all life is part of a single, self-continued process.
This, however, is no more than might be expected. If,
as we have seen to be the case, life requires substances
which do not arise in lifeless nature, and involves
processes which lifeless things cannot carry out, it is not
likely to arise spontaneously amid lifeless surroundings.
It may well be that a process so peculiar required
for its starting conditions which existed in a former
state of the earth, but cannot now be brought about.
These conditions, whatever they may have been, must
have included the presence of a factor that endowed the
first living beings with some degree of that purposive-
ness without which they could not have survived. The

1 The experiments are usually made with a broth or infusion of meat
or hay which is kept in test tubes whose mouths are plugged with
cotton wool, put in while the contents are boiling, to serve as a filter.

THE ANIMAL IN THE WORLD

727

circumstances which brought into being protoplasm in a
state of metabolism must also have brought into being
a rudimentary capacity for purposive reaction. It is
interesting to speculate whether this could arise by a
fortuitous arrangement of unorganised matter, or must be
regarded as the development of some latent tendency in
the lifeless universe. In any case purposive reaction has
evolved to greater complexity as living things themselves
have evolved, and that presumably by variation and
selection, if we accept this as the method ot evolution ot
organisms. However that may be, the facts that we have
here considered give a new importance to that ceaseless
activity by which living beings maintain their existence
amid their lifeless surroundings. If that activity failed
or were overborne, life, so far as we can see, would cease
for ever.

.

APPENDIX

PRACTICAL WORK
A. GENERAL INSTRUCTIONS

It is absolutely imperative that the student should make a careful
personal examination of each of the animals about which
Practical Work.he reads, and should verify to the utmost extent possible
the statement made in his text-books. However clearly
he may seem to have understood these statements, he will never really
compiehend any organism until he has handled it himself, nor will he
by any other means realise that his subject-matter is the living animal
and not what is said about it. In the following pages certain instruc-
tions are given to facilitate this practical work, but they must not be
considered as exhaustive, and the student should follow out any lines
of investigation which his own ingenuity can suggest to him.

The following apparatus, etc., will be needed : —

1. Some dissecting instruments , including two or three dissecting

knives or scalpels of various sizes, a large and a small
Apparatus. pair of forceps , a large and a small pair of fine- pointed
dissecting scissors, a blunt probe or seeker, and some
needles mounted in handles. A box of such instruments can
be bought for about £\, 5s.

2. A dissecting dish for dissecting under water. A shallow pie-

dish with a sheet of cork weighted with lead on the bottom
will serve this purpose.

3. A magnifying glass with a stand and arm to hold it over the

dish while both hands are used for dissection.

4. Some stout pins, a sponge, and a duster.

5. Some wide-mouthed jars with corks or stoppers to keep

specimens for dissection or from day to day while they are
being dissected. A 2 per cent, solution of formalin in water
is the best preserving fluid in most cases, but 70 per cent,
methylated spirit may be used.

6. Plenty of clean w'ater.

7. Glass pipettes. Some of these should be drawn out to a fine
opening : others should have wider mouths.

g. Chloroform for killing, and various other reagents for staining,
etc., which will be mentioned later.

729

730

APPENDIX

dissection.

7.

3*

9* A microscope with the apparatus and reagents necessary for
its use (see below).

In all cases in which it is possible, the animal should be examined
. alive before dissecting, staining, etc. The shape and

Instructions : attitudes, movements, mode of feeding, respiratory
Killing. movements, and so forth, should be carefully noted. It

must then be killed for detailed examination, which will
generally include both dissection and microscope work. Small organ-
isms may be killed with a drop of some poisonous fluid, such as alcohol,
or solutions of corrosive sublimate or osmic acid (see below). Cray-
fishes are best killed by sudden immersion foi a few seconds in boiling
water. . For most other animals the best method is the use of chloro-
form either by placing them in a closed vessel with a bit of sponge
soaked in the liquid, or by holding a cloth similarly soaked over the
nostrils. Care should be taken that the exposure is long enough to kill.

Dissection is an art that must be acquired by practice. The fol-
lowing rules will be of use to the beginner : —

I. Never start till you are sure what you are looking
for .

Never cut anything till you know what it is.

Fasten down the animal with pins to the bottom of the dis-
secting dish or with nails to a dissecting board (according to
size) and keep the organs well stretched.

4- Dissect along , not across , such structures'as nerves and blood
vessels.

5. Keep your dissecting instruments sharp. They should be

scrupulously cleaned and dried before being put away, and
fine instruments should never be used for coarse work.

6. Small animals, including the frog, should be dissected under

water , which should be changed as soon as it becomes cloudy
from the presence of blood, etc. The water supports and
keeps apart the organs, prevents their surfaces from glistening,
as they would do if they were merely damp with blood, etc.,
and helps to keep them clean. You will find it of no advan-
tage to take the animal out of the water with the object of
seeing parts of it more clearly.

Careful drawings should be made at all stages of the examination.
They should never be copied from books or lecture diagrams. The use
of coloured chalks in these drawings is not desirable, as it enables you
to represent an organ by a mass of colour without realising its outline.
Thedrawings should be of a good size in order to show detail clearly.

It is easier to draw a symmetrical object after making a faint line upon
the paper for the rniddlq of the object. .

The use of a compound microscope also requires practice. Such a
microscope consists of a stand bearing a horizontal stage
The Micro- for the object, a mirror to throw light through the object
•cop#. from below by way of a hole in the stage, a diaphragm

to vary the amount of light, a vertical tube through
which the object is viewed from above, and combinations of lenses
which are placed at the ends of the tube. Two such combinations must
be used — an objective or object glass which screws into the lower end of

APPENDIX

73*

-e

the tube, and an ocular or eye-piece which slips into the upper end.
Objectives and oculars are of various powers, and an objective of high
power may be used with an eye -piece of low power, or vice versa.
The lowest magnification obtainable in
a student’s microscope is usually about
50-80 diameters, the highest about
250-300. The object is placed upon
the stage and brought into focus by
raising and lowering the tube. Coarse
adjustment is effected by sliding the
tube, either directly or by a rack and
pinion ; jine adjustment by raising or
lowering the arm which holds the tube.

This is done by a screw which works
against a concealed spring. With the
high power the objective is closer to
the object, when it is in focus, than
with the low. An object may be viewed
either by reflected light falling upon it
from above, or, if it be transparent, by
transmitted light cast through it from
below by the mirror. The object is
placed or mounted upon a glass slide.

Usually it is immersed in some medium
which is either temporarily or perman-
ently fluid (see p. 732). In this case it
must be protected Dy a coverslip of thin
glass.

In many cases it is desirable to stain
the object. A few re-
•tamfrtM* agents, such as methylene
blue , will stain living ob-
jects ; for most it is necessary that the
animal or tissue should be killed. This
is done with a fixing agent , a strong
poison that kills rapidly and so allows
only the minimum of change to take
place in the object. Saturated solution
of corrosive sublimate in water, 2 per
cent, solution of osmic acid in water,

I per cent, solution of glacial acetic
acid in water, absolute alcohol , and other
substances, are used for this purpose.

Osmic acid is useful for small animals ;
for tissues, a mixture of nine parts cor-
rosive sublimate solution and one part glacial acetic acid is a good re-
agent. The specimen must be thoroughly washed to rid it of all traces
of the fixative before staining. Carmine , and logwood or hcematoxylin
are common stains. Various preparations of each ot these are in use for
different purposes ; they can be bought ready made, and directions for
preparing and using them may be found in books, such as Marshall and

a,,

Fig. 536. — Side view of &
compound microscope,
with detached eye-piece
and objective.

Milled rim of screw of fine ad-
justment ; b., base of stand ;
c ., collar ; d., diaphragm which
can be turned to bring between
the mirror and the object holes
of various sizes, one of which is
shown ; e.t eye-piece or ocular,
above the end of the tube into
which it slides ; m., mirror ; o ,
position of opening in stage ;
ob., objective, below the end of
the tube into which it screws ;
/*., portion of pillar fixed to
ba«e ; movable portion,

raised <or lowered by fine ad-
justment ; s., stage ; t., tube.

732

APPENDIX

II ui st s Practical Zoology . Borax carmine and hematoxylin are alcohol
stains. Piero car mine is a water stain containing picric acid. After
staining, the object must be washed with dilute alcohol (for an alcohol
stain) or water (for a water stain) to remove the excess of stain.

Small aquatic animals may be mounted alive in water ; parts of the
tissues of larger animals which are to be examined in the
Mounting, living condition must be mounted in normal salt solution—

. a '75 Per cent, solution of common salt in distilled water.
1 his approximates more nearly to the natural fluids of the bodv, and
has not the injurious effect of pure water. Objects which are intended
tor a prolonged examination should be mounted either in glycerine or in
some solid medium, such as glycerine jelly , which becomes solid when
cold, or Canada balsam, which becomes solid when dry. An object
may be placed direct from water into glycerine ; to be mounted in
panada balsam it must first be dehydrated by soaking in absolute
alcohol, then steeped in oil of cloves or in xylol till the alcohol is
removed, and then placed in a drop of balsam upon the slide and
covered. The object should not be placed direct from water into
absolute alcohol, lest the diffusion currents set up by this strong de-
hydrant should injure it, but 30, 50, 70, and 90 per cent, solutions of
alcohol should be used successively for a period varying with the size
and density of the object. Thus the complete process for staining and
mounting in Canada balsam involves the successive use of : stain to Der
cent, or 50 per cent, alcohol to wash, 70 per cent., 90 per cent., and
abso ute alcohol, xylol or oil of cloves, and Canada balsam. A very
small ooject is best submitted to these processes on the slide, either
under a coverslip (p. 731) or not, but for most they are performed in
watch-glasses.

It is often important to study slices or sections of animals, organs or
tissues. This may sometimes be done by placing the object (after
hardening in absolute alcohol, corrosive sublimate, or other hardening
agent) between two pieces of pith or carrot, and slicing off sections
freehand with a sharp razor. More often, however, it is necessary
to use a section-cutting machine or microtome. The
Smtton. object is embedded before cutting in some fluid which

cutting'. solidifies, such as parafhn wax. The technique of this

process should be learnt in the laboratory. It is
described in Marshall and Hurst’s Practical Zoology and other books.

The following hints will be of service in using the microscope : '

Mints on the I# E^amj.n<ievery object first with the low power, using
use of the power afterwards if necessary.

Microscope. 2* Focus roughly with the coarse adjustment, and only

then use the fine. 7

3. Use the utmost care to prevent the objective from getting dirty

It is damaged by cleaning. Never use the high power without
a cover glass. The eye-piece and objective may be cleaned if
necessary with chamois leather or silk. Canada balsam mav
be removed by the very careful use of benzol or xylol, but in the
laboratory it is better to leave this to the demonstrator.

4. The object may appear indistinct owing to the presence of dirt

on the coverslip, the objective, or the ocular. If the dirt be

APPENDIX

733

upon the coverslip the dimness will be affected by moving the
slide ; if it be upon the ocular, by turning the latter ; if neither
of these be in fault the objective is dirty. In a wet prepara-
tion a dirty coverslip must be replaced ; a dry one may be
cleaned in the same way as the objective.

5. If the tube will not slide freely, it or the inside of the collar

tube which holds it is dirty. They may be cleaned by careful
but vigorous rubbing with a dry duster.

6. Keep both eyes open in looking through the microscope.

The following is an outline of the procedure which should be followed
in the practical examination of an animal : —

Mow to Examine 1. Examine alive, as recommended above,
an Animal. 2, Make a drawing of the external features. These

are generally best shown in a side view.

3. Make separate drawings of parts, such as limbs, the mouth, etc.,
which are not fully shown in your first drawing. In the case
of the crayfish and cockroach this will involve several drawings
of appendages.

4- If the animal have a perivisceral cavity in which the organs lie
free, open this cavity and make a drawing of the organs in situ
without disarrangement. Vertebrates should be opened on
the ventral side, invertebrates on the dorsal. If the body-wall
be hard, as in the crayfish and cockroach, it m/iy be removed
as a single piece after two lateral incisions ; if it be soft, a
median incision should be made, and the body-wall turned
back or reflected to each side and pinned out. In a vertex
brate the skin and the muscular body-wall must generally be
turned over separately. The thorax of a mammal should not
be opened till a later stage, and the relation of the organs and
cavities should be observed carefully during the process.

Remove the alimentary canal by cutting through its ends, and
the mesentery if there be one. Draw (a) the removed canal,

( b ) the body cavity with the organs left behind. In the cray-
fish, the endophragmal skeleton must be cut away in order to
show the nerve cord.

6. In a vertebrate, make a special dissection of the heart, the great

vessels entering and leaving it, and the respiratory organs.
Make a drawing.

7. In a vertebrate, make a dissection of the organs of the neck and

throat from the ventral side. Draw.

8. Expose the brain or cerebral ganglia, and examine it in situ with

its nerves. Draw. Remove, and draw dorsal and ventral
views. Ninety per cent, alcohol is useful in whitening and
hardening the nervous system.

9. In a vertebrate, make a series of drawings of the parts of the

skeleton. Skeletons may be prepared by removing most of
the flesh and boiling or macerating (i.e. placing in water and’
allowing the flesh to rot). They should then be washed
thoroughly and bleached in sunlight. Cartilaginous skeletons*
however, should not be rotted or allowed to dry, but dipped
into hot water for a few minutes and cleaned with a brush.

734

APPENDIX

10. Examine under a microscope and draw: (a) some of the
blood ; (b) if possible the sperm ; {c) in the case of at least one
of the vertebrate animals studied, portions of the tissues fresh

f n ? aTe l - .m ^be case small animals, transverse and
longitudinal sections of the whole body.

11 Ehavene and draW any larVal stages tbat animal may

The following pages contain directions for observing and dissecting
accord mg to the instructions given above, a number of animal type!.’

itftahlW?rkmSi th.\°Ugh these in or three cases, the student should
be able to apply the same general directions to the examination of other
animals which he may wish to study.

B. INSTRUCTIONS FOR OBSERVING AND
DISSECTING CERTAIN ANIMALS

The nu,nbtrs of the items refer to the General Instructions on

PP* 733' 734-

Frogs are easily found in damp places during a great part of the year
The Frog. but in late autumn and in winter they can most con'
i r venifntIy be obtained from persons who have a stock of

Wb«nat?on.fr°m lh°Se W Sk'lled ™ finding them du™8 thei'

1. Observe how the animal sits, crawls, jumps, and swims ; the
movement of the floor of the mouth in breathing ; aid if

coupled He feedlng' At the breeding season look for pairs

Kill by enclosing in a vessel with a little chloroform.

2. In a side view, note: head and trunk (no neck or tail); fore

and hind limbs ; mouth, nostril, eye, tympanic membrane
(pierce and pass seeker through Eustachian tube to mouth) •
sacral prominence (Fig. 12). *

Find cloacal opening.

In hand and foot, number the digits. Note: palmar and
plantar surfaces ; absence of claws or nails ; web of foot •

, Pad °n first Ceall7 second) digit of hand in male (Fig. n).

' °r^he u °Utf Wldely- If the gullet rise, press it back with
the handle of a scalpel. Note: maxillary and vomerine

V Postenor ”arcs; eyeballs; Eustachian tubes; tongue •
its attachment ; glottis (Fig. 537). b ’

Lay the frog on its back under water in a dissecting dish •

thT ^°Ugh feen and dp °f jaW ; cut the sk™ along

the mid-ventral line, thus opening the ventral lymph sac-

free the skin where it is held down on the breast, turn it

outwards, and pin. Note : rectus abdominis, pectoral, and

v A!1 the animals ordinarily studied in the laboratory can be obtained fmm d.„i
w W adverusement, are usually to be found in the pages of Nature ^

3«-

ta.

APPENDIX

735

submaxillaris muscles ; xiphoid cartilage ; anterior abdo-
minal and cutaneous veins ; hypoglossal nerves (Fig. 26).
b. Cut through the muscles of the ventral body- wall, and
through the pectoral girdle, on one side of the middle line.
Ligature doubly the anterior abdominal vein near the liver ;
cut between the ligatures ; carefully cut through the muscles
which still hold down the pectoral girdle ; turn back the
body-wall, and pin it. Arrange the viscera for drawing to
show as much as possible, noting : heart ; lungs, liver,
gall bladder, stomach, small intestine (duodenum and
ileum), rectum, bladder, pancreas, hepato-pancreatic duct ;

Fig. 537. — The mouth of a frog.

ft, Protuberance caused by eyeball; e u., Eustachian tube; gls., glottis;
i.n., internal narial opening ; m.t., maxillary teeth ; me/., lower jaw ;
t., tongue; v.t., vomerine teeth.

spleen ; ovaries or testes, fat bodies, and, in female,
oviducts (Fig. 32).

5<2. Remove the alimentary canal by cutting through the
oesophagus, mesentery, and rectum. Cut open the intestine,
wash, and note folds of wall.

b. In the abdominal cavity, note : fat bodies, testes or ovaries,
kidneys ; kidney ducts ; in male, vesiculse seminales and
vasa efferentia ; cloaca, bladder ; and the following veins —
femoral (traced backwards for a short distance by carefully
snipping through connective tissue and parting muscles at
back of thigh), renal-portal, sciatic, pelvic, vesical, and
anterior abdominal (cut) (Figs. 43, 45).

736

APPENDIX

6 and 7a. In another frog, or before doing number 5, open the peri-
cardium, turn the heart forwards, and cut the ligament which
connects the ventricle dorsally with the pericardium. Note :
glossopharyngeal, vagus, and hypoglossal nerves ; ventricle,,

$m.

Fig. 53S. — The forepart of a frog’s body dissected from the ventraf
side, and stretched to display the organs.

cgL, Carotid gland; car., common carotid artery; cu., cutaneous artery;,
ej., external jugular vein; hy., hypoglossal nerve; i.j. , internal jugular
vein; l.a., lingual artery; l.L, left lung; l.v., lingual vein; lr., liver;
md., mandibular vein; ov., part of ovary; p., pulmonary artery; s.scp .,
subscapular vein ; scL, subclavian vein ; sm., submaxillaris or mylohyoid
muscle ; si., stomach ; sy.a., systemic arch ; thr., thyroid gland ; v., vent-
ricle; IX., glossopharyngeal nerve ; Xc., Xg., Xlar., Xp., cardiac, gastric,
laryngeal, and pulmonary branches of vagus nerve.

auricles, and sinus venosus ; inferior vena cava and hepatic-
veins ; superior venae cavae, external jugular, innominate,
internal jugular, subscapular, subclavian, musculo-cutaneous,
and brachial veins; pulmonary veins (Fig. 538; but im
this figure the heart is in its normal position and the
pericardium unopened).

APPENDIX

737

The heart will sometimes be beating when the body is
opened, though the animal is dead. If not, it may often be
stimulated to beat by a prick. In the beating heart, note
the successive phases of the beat (p. 68).

7 b. Turn the heart back, cut through one superior vena cava,
and remove its branches. Wash the dissection. Note :
auricles, ventricle, truncus. Trace : carotid, systemic, and
pulmonary arches : lingual (external carotid) artery, carotid
gland, and internal carotid artery ; subclavian and coeliaco-
mesenteric arteries ; dorsal aorta (Fig. 40;.

Sa. Remove the kidneys and generative organs without injuring
the aorta. Note : vertebral column ; dorsal aorta and

FiG, 539- — a., A diagram of part of a section of the testis of
a frog, showing one ripe seminiferous tubule and portions
of two others, with connective tissue binding them together,
and blood vessels [b.v. ). B., A ripe spermatozoon.

I.C., Cells of the lining epithelium. Some of these are undergoing the
divisions by which they form the spermatocytes from which in turn
spermatozoa arise ; s.c., supporting cells, carrying sp., bundles of
spermatozoa.

iliac arteries ; spinal nerves, with calcareous concretions ;
ganglia and commissures of sympathetic chains ; rami com-
municantes ; sciatic plexus (Fig. 47 ; see p. 86).
b. Remove the roof of the skull and the neural arches. Harden
the central nervous system by pouring on two or three
changes of 90 per cent, alcohol. Note : olfactory lobes,
cerebral hemispheres, thalamencephalon, optic lobes,
medulla oblongata, fourth ventricle, spinal cord (Fig. 50, I).
<. Remove the central nervous system and observe its ventral
side. Note : olfactory lobes, cerebral hemispheres, optic
chiasma, infundibulum, pituitary body (if not torn away),
crura cerebri, spinal cord (Fig. 50, II).

9. Examine and draw the skeleton in the following divisions :

4/

7.3$

APPENDIX

backbone and pelvic girdle in dorsal view (Fig. 15) ; skull
in dorsal (Fig. 15) and in ventral view (Fig. 17); hyoid
(Fig. 21); "fore-limb (Fig. 15); hind-limb (Fig. 15);
pectoral girdle in ventral view (Fig. 22) ; pelvic girdle in
side view (Fig. 23).

10 a. Examine a drop of the blood diluted with normal ( 75) salt
solution (Fig. 81, A).

b. Tease in normal salt solution a piece of testis. Note sper-

matozoa (Fig. 539, B).

c. Examine in normal salt solution the following : pigment cells

in web of foot (Fig. 72, B) ; striped muscle hbres, frayed
apart (Fig. 77) ; unstriped muscle hbres, in wall ot bladder
spread out (Fig. 76) ; connective tissue, in thin sheet as
found among leg muscles (Fig. 79) ; fatty tissue, from tat
body (Fig. 80) ; cartilage, from edge of xiphisternum scraped
clean (Fig. 78) ; nerve hbres, obtained by fraying out a
large nerve with needles (Fig. 70).

d. Examine the following sections prepared by an expert :

bone (Fig. 7) ; nerve (Fig. 71) ; spinal cord (Fig. 49) ;
intestine (Figs. 58, 59) ; testis (Fig. 539, A).

e. Pith a frog (i.e. cut through the backbone just behind skull,

and pass a seeker through the wound forwards into the skull
to destroy the brain and backwards into the neural canal).
Arrange the animal so that one toot lies with the web
spread out under the objective of a microscope. \\ ith
the low power note the circulation in the capillaries (Fig.

3$).

/. Skin the leg of the same frog. Stimulate the gastrocnemius
muscle (Fig. 26) — (1) by pinching, (2) by touching with
a hot needle, (3) by a drop of dilute acid. Note contrac-
tion of the muscle in consequence ot these mechanical,
thermal, and chemical stimuli.1 Dissect out the sciatic
nerve (which may be traced back trom the abdominal
cavity) and apply to it the same stimuli. Note contraction
of muscles.

11. In the spring, obtain some spawn and preserve a portion of it
in 2 per cent, formalin solution or 70 per cent, alcohol.
Keep the rest in a large vessel of water with living water
weeds. Watch the hatching and growth of the tadpoles,
preserving some at every stage. Examine carefully and
note: cleavage (Fig. 469) ; gastrulation (Fig. 471) ; and the
history of the neural folds, sucker, mouth, gill-clefts, gills,
operculum, limbs, and tail (Figs. 11, 473, 475, 470, 47$).

Amoeba proteus may be bred from the surface layer of clean pond-
mud which contains its minute food-plants. This is allowed to
stand in tap water. A brown scum arises and is poured oft with
the upper water and a few wheat grains added to it. Minute amoebte

1 This experiment does not prove the irritability of the muscle fibres independent
of the nerve endings upon them unless the latter have first been numbed, as by
the injection of the poison curare into a lymph sac of the frog.

APPENDIX

739

appear, grow, and divide. The species of Entamoeba which is most
convenient for laboratory purposes is E. blattce , found among the

contents of the rectum of the cockroach.
In cold weather, the cockroach should be
obtained from some warm place, such as a

Amoeba Entamoeba,
Polytoma, Euglena,
Chlamydomonas, Opalina

Parameculum, Balantidium, bakehouse. Polytoma occurs with allied
Nyetoiherus, Vorticella, organisms in water which is very foul with

onocys is. decaying animal matter, as in the macerating

tub in which bones are prepared for mounting as skeletons ; Euglena
in the green water which sometimes fills small ponds and hoof-prints ;
Chlamydomonas in water butts and ponds where the water is greenish ;
Opalina , Balantidium, and Nyctotherus in the upper part of the
rectum of the frog ; Paramecium in hay infusions or similar solutions
of decaying vegetable matter (p. 161). Vorticella may be found
plentifully growing on objects in ponds and streams, as, for instance,
on willow roots. Monocystis will generally be found on opening the
vesiculae seminales of a well-grown earthworm, the large species
hanging on to the funnels of the vasa deferentia as white threads
visible to the naked eye, the smaller more often free in the contents
of the vesicular, which must be mounted and examined, to find also
the cysts, with various stages of conjugation and spore formation.

1. Examine each of these Protozoa alive, {a) with the naked eye,

if possible, to note the true size, (b) with low power, (c)
with high power. Free living forms should be mounted in
water ; parasites and Polytoma in the fluids in which they
live, diluted with a drop of normal salt solution. Para-
mecium may be made to move more slowly by the addition
of a little gum to the water in which it is mounted.
Polytoma is quiet in the drying edges of a film of the fluid
which contains it, mounted without ccverslip.

2. Kill by placing at one side of the coverslip a drop of osmic

acid and drawing under with blotting-paper at the opposite
side, taking care not to sweep away the animals. Wash
with water in same way, stain in same way with picrocar-
mine, wash again, draw under coverslip glycerine, first
dilute and then pure. Flagella and trichocysts are best
shown by using iodine solution, which both kills and stains.
(. Polytoma is best fixed with osmic vapour in a film on a
coverslip, stained there, and subsequently mounted.)

In living and stained specimens, note : shape, changing in Amoeba
and Entamoeba by amoeboid movement, in Euglena and Monocystis by
euglenoid movement ; pseudopodia, flagella, cilia, or absence of all these :
nucleus (nuclei in Opalina ) ; micronucleus in Ciliata except Opalina ;
stainihg of nuclei; ectoplasm, endoplasm, food vacuoles (not in Chlamy-
iomonas , Polytoma , Euglena , Opalina , or Monocystis ) ; contractile
vacuole or vacuoles (none in Monocystis or Entamoeba ), whether simple
or complex ; trichocysts in Paramecium ; red spot and chloroplasts in
Euglena &n<\Chlamydomonas’, gullet and vestibule in Ciliata other than
Opalina ; feed these Ciliata with Indian ink ground up in water ;
stages of Monocystis. (Figs, io, 88, 90, 94, 96-98, 101,110, 111,128, 129.

740

APPENDIX

Specimens of one or more species of Hydra may usually be obtained
by gathering weeds from a clean pond and allowing them
Hydra. to stand in a vessel of water. Some of the polyps will

migrate to the wall of the vessel, where they can easily
be detected, gently pushed off, and sucked up with a pipette.

I a. Note: extension, contraction on touching, spontaneous
movements.

b. Place a brown specimen with a water-flea in a watch-glass
containing only a little water. Note that the water-flea is
numbed when it touches the tentacle of the Hydra.
Probably it will be seized and swallowed by the latter.

2. With hand lens, note : body ; foot ; tentacles ; bud, testes,

and ovary, if present (Figs. 138, 146, 147).

3. Mount in water, raising coverslip on two small strips of paper.

Note : with low power, oral cone and mouth ; with high
power, in tentacle, ectoderm, endoderm, batteries of
nematocysts, cnidocils (Fig. 140) ; after running 5 per cent,
salt solution under coverslip, threads of nematocysts
extruded (Fig. 143, D).

Small organisms often found running over the body of the
Hydra are ciliate Protozoa.

10. In stained and mounted sections (transverse and longitudinal),
note, under low power ; ectoderm, endoderm, jelly, enteron
(Figs. 142, 139) ; under high power, details of ectoderm
and endoderm (Fig. 144).

On rocky parts of the coast Obelia and other hydroids may be
found at low tide. They will travel alive in a vessel of
Obalia. sea water.

1. Note retraction of polyp heads when touched or

2.

10.

11.

shaken.

Kill a twig by dropping osmic acid suddenly on to it when it is
in a small quantity of water, with tentacles expanded. Stain
and mount. Note : hydranths, wdth tentacles and mouth,
coenosarc, gonangia, peritheca, hydrothecae (Figs. 149, 150).
Examine a vertical section of a hydranth (Fig. 1 5 1 ) .

Examine a medusa, stained and mounted (Fig. 154).

Fresh specimens of the Liver Fluke can only be obtained when a
sheep infected with them is killed. Laboratory speci-
mens are usually preserved in formalin.

2. Note : mouth, ventral sucker, genital pore
(Fig. 165).

Dry the surface of the body and under a lens note spicules.

In a specimen which has been fixed while it was flattened
under pressure and mounted in balsam, note : pharynx,
intestinal caeca ; testes, cirrus sac ; ovary, yolk glands,
uterus, shell gland (Figs. 167, 169).

In an injected specimen observe excretory systems (Fig. 167).
Examine a transverse section through the region of the ovary
(Fig. 166).

Stages of the life history should be examined if available
(Fig. 170).

The Liver
Fluke.

2.

4-

5-

10.

11.

APPENDIX

741

Fresh Tapeworms are rarely obtainable. Laboratory specimens
are preserved in formalin.

The Tapeworm. 2. Note : head, neck, proglottides (Fig. 173, 5).

3. In head, under magnification, note : suckers,
hooks (Fig. 173, 4).

4 a. In young proglottides, stained and mounted, note : excretory
canals ; testes and vas deferens ; ovary, uterus, vagina
(Fig. 174).

b. In old proglottis, note : large, branched uterus, with thick-
shelled fertilised eggs (Fig. 173, 6).

10. In transverse section of a proglottis, note : cuticle, nuclei of

ectoderm, withdrawn into deeper parts of cells, lying in
superficial regiqn of parenchyma ; longitudinal muscle
fibres ; . layer of transverse muscle ; excretory canals ; nerve
cords ; testes, with developing spermatozoa ; uterus, with
fertilised eggs; yolk glands (Fig. 175).

11. In cysticercus, note: head with hooks and suckers, young

proglottides ; bladder (Fig. 173, 3).

Fresh cysticerci of a tapeworm which infects the dog may often be
found in the mesentery of the rabbit. On being placed in warm salt
solution, some of them will evert the head.

If the Earthworm be studied during the winter, advantage should
be taken beforehand of good weather for obtaining
The Earth- specimens. They can be kept alive in damp leaves
worm, and earth for some weeks.

:v' -- 1. The movements and habits of earthworms are

best studied in the field. Darwin’s Earthworms and Vege-
table Mould is an interesting work on this subject.

2. Note : in dorsal view, prostomium, peristomium, following

somites, clitellum, anus (terminal) ; in ventral view, mouth,
chsetse, genital pores (Fig. 178).

3. Extract a chaeta with forceps and examine it under low power

(Fig. 180).

4. Pin the worm ventral side downwards and stretch it well.

Open it by a cut along the dorsal middle line from the
prostomium to somite 25, turn the body-wall outwards,
snipping through the septa, and pin. Note : septa ;
nephridia ; suprapharyngeal ganglia ; mouth, pharynx,
oesophagus, crop, gizzard, intestine ; vesiculae seminales ;
spermathecae (these mark somites 9 and 10, and thus enable
the other somites to be identified) ; dorsal blood vessel,
hearts, contractions of these if the worms have been recently
chloroformed (Figs. 184, 540, diagram).

5 a. Cut through the gut just behind the gizzard, lift it with
forceps, and working forwards separate it from the septa.
In somite 13 note the ovaries before cutting the septum
in front. Be careful not to remove the vesiculae seminales.
Cut through the pharynx behind the suprapharyngeal
ganglia and remove the alimentary canal. Note the
parts mentioned above, and also oesophageal glands and
pouches.

742

APPENDIX

b. In the worm, note the following structures exposed by-
removal of the alimentary canal : suprapharyngeal ganglia,
circumpharyngeal commissures, subpharyngeal ganglia,
ventral nerve cord ; spermathecae, vesiculse seminales,
ovaries ; nephridia (Figs. 183, 193).

10 a. Open another worm under normal salt solution.

Remove a nephridium with the part of the septum in front
of it to which it is attached, mount it in salt solution, and
examine it under the microscope. Note : nephrostome
(Fig. 187) ; various portions of tube with capillaries (Figs.
186 and 188). Small, parasitic nematode worms will
probably be seen.

Remove and mount an ovary .(Fig. 192).

Mount contents of a vesicula seminalis. Examine alive,
fix with a drop of absolute alcohol, stain with picrocarmine

cL.br: ht.

Fig. 540. — A diagram of the principal blood vessels of the earthworm.

d.b.v., dorsal blood vessel ; d.s.v., dorso-subneural vessel ; ht., one of the “ hearts ” ;
ini., intestine ; m., month ; ces., oesophagus ; ph., pharynx ; sup.ph.g., supra-
pharyngeal ganglion ; s.i.v., subintestinal vessel ; s.n.v., subneural vessel ;
v.n.c., ventral nerve cord.

(p. 732), and examine. Observe stages of development of
spermatozoa (Fig. 191).

Mount and examine contents of a spermatheca. Examine
and note ripe spermatozoa (Fig. 191).
b. Examine under low power a transverse section of the intestinal
region. Note : in body-wall, cuticle, epidermis, and circular
and longitudinal muscles ; coelom ; in gut-wall, typhlosole
chloragogen cells, muscular layer, endoderm ; nerve cord ;
dorsal subintestinal, and subneural blood vessels ; nephridia ;
chsetse (Fig. 181). Examine portions of the section under
high power (Fig. 186).

Longitudinal sections of the earthworm are difficult to interpret
because the worm is never quite straight when it is cut, and because
the contents of the section vary with the distance from the middle line,
but there may be found in them, besides the body-wall, some or all
the regions of the alimentary canal, septa, vesiculae seminales, and
usually portions of the dorsal blood vessel, ventral nerve cord, and
suprapharyngeal ganglia (Fig. 182, diagram!.

APPENDIX

743

Specimens of the Leech must be bought from a dealer.

I. Note the movements of the worm in swimming and
The Leech. looping. It is often difficult to induce it to feed.

When it does so, it can be caused to relax its hold
by salt sprinkled on its back, but the bleeding of the wound
is sometimes difficult to stop. Kill with chloroform.

2. Note : in dorsal view, annuli, spots which mark the true

somites, eyes, hinder sucker, anus ; in ventral view, oral
sucker, mouth, penis (thrust out), female opening, hinder
sucker (Fig. 198).

3. Examine the mouth and remove a jaw (after 5).

4. Pin the leech through the suckers, stretching well. Open it

by a dorsal incision a little to one side of the middle line ;
carefully dissect away the skin and pin it out. Wash.
Note : dorsal sinus, lateral vessels ; pharynx, crop, caeca,
stomach ; intestine (Fig. 199) ; nephridia.

5. Remove the alimentary canal. Note : nervous system ;

nephridia ; generative organs (Fig. 198).

10. Examine a transverse section (Fig. 200).

Crayfish occur in streams in many parts of Britain, but are most
conveniently obtained from dealers. Bought specimens
The Crayfish. may be of the French species.

I. In the living animal note movements of limbs in
locomotion and feeding. With a pipette place carmine at
the bases of the legs and note that it is sucked under the
branchiostegite and driven out below the antenna. Cut
away the branchiostegite above the jaws, and note the
movement of the scaphognathite.

Crayfish may be kept alive for a long time in running water. They
should be killed with hot water, and may then be preserved in 70 per
cent, alcohol (renew the, spirit). Preserved specimens are only of use
for examination of external features' and of the nervous system.

2. Note : in dorsal view, cephalothorax, rostrum, cervical groove.

branchiostegite, abdominal terga, telson ; eyes ; antennules,
antennae, scales of latter, great chelae, walking legs, uropods
(Fig. 201) ; in ventral view, limbs (working with list on
p. 296), as individual jaws are difficult to identify for the
first time until they are removed) ; sterna (Fig. 202).

3. Remove the telson. Note the sixth abdominal somite with

its limbs.

Remove the sixth abdominal somite and view the fifth
from behind. Note : tergum, sternum, pleura, limbs (Fig.
2°7).

Remove the limbs of one side from behind forwards,
bringing away the podobranchiae with the legs to which they
belong. Arrange the limbs on a sheet of paper, and compare
with Figs. 203-207, noting the parts of each limb.

Cut away a branchiostegite (Fig. 212) ; count gills (p. 307).

4. Cut away the median part of the carapace and of the abdom-

inal terga. Note : heart, ostia ; ovary, or testes and vasa
deferentia ; “ stomach,’’ muscles running from it to cara-

744

APPENDIX

pace, “ liver,” “ intestine ” ; mandibular muscles, longi-
tudinal muscles (Fig. 209). _ .

In an injected specimen,1 similarly opened, note . ophthal-
mic, antennary, gastric, and dorsal abdominal arteries
(Fig. 209). Press apart organs and note : hepatic arteries,

sternal artery (shown in Fig. 210).
ca Remove the heart and genital gland. Cut through the
oesophagus and hind gut and very carefully remove the
alimentary canal under water. In the removed canal note :
oesophagus, “ cardiac ” and “ pyloric ” regions, mesenteron,
with caecum, “ liver,” joining mesenteron by a short duct,
hind gut (Figs. 208, 210). Q

b. Examine the removed genital gland and its ducts 'figs. 210,

10.

II

21 9). -i

c. Remove carefully the longitudinal muscles, leaving the nerve

cord behind. Cut through the false bottom of the thorax.
Whiten the nerve cord with 90 per cent, alcohol. Note :
green glands, with their sacs ; cerebral ganglion, circum-
oesophageal commissures, suboesophageal ganglion, ventral
nerve cord, with its ganglia ; sternal artery (Fig. 215).
Examine under a microscope the contents of a vas deferens
(spermatozoa, Fig. 220), and a muscular fibre (striped).

In a “ berried ” female, note how the eggs are attached. It
possible find a female carrying young and examine them
(Fig. 221).

Cockroaches are easily caught, in places they intest, with traps sold
for the purpose. They should be killed with chloroform,
the Cockroach. I. In a living specimen, note the mode of use of the

legs and antennae, the movements of the jaws in
feeding, and those of the abdomen in respiration.

2. Note : head, neck, thorax, abdomen ; segments of
eyes, antennae, wings, legs, and cerci, and in male
terga and sterna ; stigmata (Figs. 227, 228).

Note parts of head in side and front views (Fig. 229).

Remove and examine head appendages (big. 230c
These are best seen after boiling in potash, to remove soft
tissues, and mounting in balsam.

Examine the parts of the hinder end of body in male and in
female (Figs. 227, 240, 241).

Cut off the wings. Fasten the animal down by pinning or
(better) by embedding its under surface in melted paraffin
wax and when cool pinning through the wax. Cut through
the abdominal and thoracic terga and remove them carefully
so as not to injure the heart. Note : heart in pericardium ;
fat body ; tracheae (Fig. 541).

4$. Free alimentary canal trom tat body and tracheae and unravel

latter ;
styles ;

3a-

b.

c.

4a.

it.

Note

oesophagus, crop, proventriculus, chylific

1 The blood vessels of a freshly killed crayfish may be injected by removing care-
fully a small piece of carapace just over the heart, placing into one ostium the point
■of a fine pipette full of carmine, and pressing gently but steadily. The point of the
pipette must not contain bubbles of air.

APPENDIX

745

5-

10.

The

Ascaris.

ventricle, ileum, colon, rectum ; salivary glands and
receptacles; pyloric caeca; Malphigian tubes; cerebral

ganglion (Fig. 237). , i .. 1 j 1

Remove stain, and mount in balsam a salivary gland and

receptacle, and also remove and mount a portion of fat body

with tracheae. . . ,

Remove the alimentary canal. Note : cerebral, subcesopha-
geal, thoracic, and abdominal ganglia ; and the commissure,
connecting them ; in females
colleterial glands, oviduct,
spermatheca, ovarian tubes
(Fig. 237) ; in immature male,
testis, vas deferens, mushroom-
shaped gland ; immature male,
mushroom gland, its duct,
accessory gland.

Examine under a microscope :
the stained salivary gland ; the
fatty tissue with tracheae (Fig.

239) ; an ovarian tube (low
power) ; contents of mush-
room gland ; muscular fibre
(striped).

most convenient source trom
which to obtain specimens
of Ascaris is the pig,
which is infested by a
species closely similar to, if not identical
with, A . lumbricoides . The related but
larger A. megalocephala from the horse
is also often available. The worms
should be preserved in formalin..

2. Note, in specimens, of the two

sexes : smooth cuticle, anterior
and hinder ends, “ excretory ”
pore, genital pore (Fig. 259).

3. Examine under a lens the ends

of the body. Note : lips,
papillae ; curved tail of male,
with penial setae ; tail of
female merely convex, without setae (fig. 200).

4. Cut open the worms a little to one side ol the middle line, and

pin out the body-wall. Unravel the genital organs Note :
pharynx, intestine, rectum ; lateral lines ; dorsal and central
lines ; in male, testis, vas deferens, penial setae m sheaths ;
in female, ovaries, oviducts, uteri, vagina (Figs. 204, 265).
In a transverse section of the intestinal region, note : cuticle,
epidermis, muscle layer ; lateral lines ; dorsal and ventral
lines ; intestine ; ovary or testis ; uterus or vas deferens

^Observe a muscle fibre under high power (Fig. 262).

Fig. 54I. — The heart and
neighbouring structures of
a cockroach : somewhat
diagrammatic.

a.rn., Areas marked by dotted lines
to show the position of alary
muscles below the tatty body ;
/.b., fatty body; h t., heart;
tr., tracheae.

JO.

746

APPENDIX

Mussels may easily be found by searching the shallow part of ponds
and streams, usually partly buried in the mud.

The Swan I. Note protrusion of foot, and current through

Mussel. siphons (Fig. 276, A).

2a. Note : in side view, umbo, lines of growth, position
of hinge, ends, of shell (Fig. 276, A) ; in dorsal view, umbo,
lines of growth, ligament (Fig. 277).

b. Remove left valve of shell by separating it from edge of mantle

and cutting through attachment of adductor muscles (keep
edge of scalpel, close to shell). Note: mantle lobes, their
thick edges, siphons, two adductor, two retractor, and
protractor muscles (Fig. 278, left mantle lobe cut away).

c. On inside of shell, note : marks of attachment of muscles,

pallial line (Fig. 276, B).

d. Turn back left mantledobe. Note : foot ; labial palps ; gills,

line of attachment of outer gill to mantle, siphons (Fig. 278).

e. Pin back labial palps and gills of left side. Note : attachment

of inner lamella of inner gill ; seeker passed between foot
and free middle part of inner gill into epibranchial chamber
and out at dorsal siphon. Left kidney and genital openings
exposed by cutting through attachment of inner gill plate
to foot (Fig. 279).

3. In end views of animal note : mouth, labial palps (Fig.
283, B) ; siphons, cloaca, anus.

4* Remove carefully dorsal wall ot body, thus opening peri-
cardium. Note : rectum ; ventricle, auricles (Fig. 282).

5. C ut through auricles and hinder end of rectum and turn latter
forwards. .Note : renopericardial openings. Cut through
door of pericardium on one side, thus opening non-glandular
limb of kidney. Note : opening to exterior ; glandular
limb of kidney ; visceral nerve-commissures.

The alimentary canal should be studied in a specimen in
which it has been injected and dissected out (Fig. 285).

8. Find nerve ganglia — cerebral by gently scraping skin behind
anterior adductor, pedal by cutting vertically through
muscular part of foot and separating the halves,
parieto-splanchnic on ventral surface of posterior adductor/
10a. Remove from its shell a spirit-preserved specimen. With a
sharp scalpel make the following sections : (1) through

middle of foot, (2) through hinder part of foot, (3) behind
foot. Note: attachments of gills (Fig. 281); internal
organs in transverse section (Fig. 2S4. ' The first section
will probably not show the heart).

11. Examine larvae if present (Figs. 287, 28S).

The edible snail (Helix pomatia) occurs in many parts of England,
and may be obtained from dealers. It is large/ than
he Snail. the common snail (H. aspcrsa). Snails are best killed
by drowning, since they then die extended.

1. Observe alive. Note : mode of walking, etc.

2a. Note : shell, lines of growth, umbilicus (opening of colum-
ella) ; head, anterior tentacles, posterior (oculiferous)

APPENDIX

747

tentacles, mouth ; foot, pore of pedal gland ; genital pore ;
collar (edge of mantle) ; opening of lung (Fig. 290,. A).

2b, Carefully cut away shell. Note : opening of lung ; anus ;
columellar muscle ; roof of lung ; visceral hump, liver,
albumen gland, kidney, pulmonary vessels (seen through
skin).

3a. Pin the snail down under water. Open the lung by a trans-
verse incision behind the collar and an incision along the
left side of the roof. Turn back the mantle roof and pin
it down. Note : floor of lung, rectum, kidney duct, root
of lung, pulmonary vessels, kidney, pericardium, auricle,
ventricle.

3 b. Open pericardium, find renoperica,rdial opening (near the
middle of the renal sidfe), and pass a bristle through it.

4 a. Cut along the right side of the roof of the lung, leaving the
rectum on the roof. Turn back the roof. Make a median
incision from the head to the visceral hump, thus opening
the haemococlic perivisceral cavity. Turn back and pin
the flaps thus made. Carefully remove the thin skin of
the visceral hump. Unravel the contained organs, pinning
out the genital organs to the animal’s right, the alimentary
canal to the left. Note : hermaphrodite gland, herma-
phrodite duct, albumen gland, common duct with passages
for ova and sperm, free oviduct and vas deferens, vagina,
mucous glands, dart sac, spermatheca, spermathecal duct
(with appendix in H. aspera), penis, retractor muscle of
penis, flagellum of penis (Fig. 290, C, diagrammatic).

4 b. Remove dart from dart sac and examine it.

,5 a. In alimentary canal, note: buccal mass, oesophagus, crop,
stomach, intestine, rectum, salivary glands, salivary ducts,
liver, liver ducts ; nerve collar (Fig. 290, C, diagrammatic),
$b. Cut open buccal mass, remove radula, mount it, and examine
it under low power. .

8a. Observe nerve collar in situ. Note : cerebral ganglion, with
nerves to buccal ganglia and tentacles, infracesophageal
mass of ganglia, with nerves to viscera and foot.

8b. Dissect away connective tissue around infracesophageal mass.
Note : pedal, pleural, and visceral ganglia.

Starfishes may be found almost everywhere upon the British coast.

They may be obtained from the Marine Biological
The Starfish. Station at Plymouth.

1. In a living specimen, note the movements of the
tube-feet, spines, and pedicellariae, and watch the animal
walking. Observe the dermal gills and the eyes. Lay the
animal upon its back and note how it rights itself. While
it is in that position, observe the organs in a widely opened
ambulacral groove. Touch the bottom of groove with a
seeker, and watch the contraction of the groove and its
protection by the adambulacral spines (Fig. 292).

The starfish may be preserved in formalin or 70, per cent.

748

APPENDIX

alcohol. Openings should be made at the sides of the
arms to admit the fluid.

2. Make drawings of the oral and aboral sides, showing the mouth,
ambulacral grooves, tube-feet, adambulacral and other
spines, and madreporite (Figs. 291, 292).

3 a. Find the nerve cord at the summit of the ambulacral groove.

b. Examine under a lens a small portion of the aboral surface.

Note : spines, cushions, pedicellariae, gills (Fig. 291).

c. Note the pedicellariae upon the adambulacral spines, between

the spines of the aboral surface, and on the cushions at the
base of the spines. Remove, mount, and examine pedi-
cellariae from each of these positions (Figs. 291, 292).

4 & 5. Cut through the body-wall along the outline of the animal, so
as to divide it into oral and aboral halves. Cut through
oesophagus, stone canal, and retractor muscles, and remove
the aboral half of the body-wall with the alimentary canal,
Note and draw : in oral half, axial sinus with stone canal and
axial organ, nerve ring, position of water vascular ring,
Tiedemann’s bodies, ambulacral ossicles, and ampullae of
tube-feet (Fig. 297) ; in aboral half, stomach, pyloric sac,
pyloric caeca, rectal caeca, and generative organs (Fig.
294).

10. Examine and draw a transverse section of the decalcified arm of

a small specimen, cut by an expert. Note: body-wall, spines,
gills, ambulacral ossicles, nerve cord, radial water vessel,
ampullae, tube-feet, body-cavity, pyloric caeca (Fig. 296).

11. If possible, examine and draw larvae (Fig. 298, simplified).
Specimens of Amphtoxus may be obtained from dealers.

2a. In a left side view, note : mouth, oral cirri ; anus ;
The Lancelet. atriopore ; myotomes ; dorsal, caudal, and anal

fins (Fig. 302).

b. In a ventral view, note : ventral groove ; metapleural folds ;
mouth, etc., as before (Fig. 303).

4. In a specimen which has been macerated tor twenty-four hours

in 20 per cent, nitric acid, remove on the right side the wall of
the atrial cavity, the muscles dorsal to it, and the oral hood.
Note : velum, enterostome, oral cirri, pharynx, gill slits,
liver, intestine, anus ; notochord ; spinal cord ; fin rays ;
generative organs.

5. Remove and stain the hind end of the pharynx, slitting it open

dorsally and flattening it. Note : primary and secondary
bars, skeletal bars, synapticulse, developing slits (Fig. 307).
8. Remove the spinal cord by shaking the macerated specimen in
a test tube with water, stain, and mount. Note : eye spot ;
first pair of nerves ; dorsal and ventral nerves ; cerebral
vesicle, central canal (Fig. 314).

no. In transverse section, note : in the pharyngeal region, myo-
tomes ; notochord and its sheath ; spinal cord ; pharynx,
gill-slits, endostyle, dorsal groove; ventral aorta and supra-
branchial arteries ; atrial cavity ; coelom ; gonads ; liver ;
metapleural folds ; dorsal fin (Fig. 30S) ; in the intestinal

APPENDIX

749

region, myotonies, notochord, and spinal cord ; intestine ;
dorsal aorta, subintestinal vessel ; anal fin (Fig. 310).

11. Examine either specimens or wax models illustrating the
development (Figs. 461-466).

The Dogfish will usually not be procurable alive.

It can sometimes be observed in an aquarium.

The Dogfish.1 2 a. In ventral view of specimen (whose belly may

have been opened to let in preserving fluid)
note : nasal openings, oronasal grooves-
mouth ; cloacal opening ; pectoral fins, pelvic
fins, with claspers in male ; anal and caudal
fins (Fig. 326).

b. In side view, note : head, trunk, and tail ; eye, mouth,,
spiracle, gill-clefts ; lateral line ; pectoral, pelvic, dorsal,,
caudal, and anal fins (Fig. 315).

3 a. Strip off a small portion of skin to show myomeres (Fig. 317).

b. Examine scales under lens (Fig. 316).

c. Examine teeth.

4. In a ventral view of the contents of the abdominal cavity,
note : suspensory ligament, with internal opening of oviducts,
in female and vestige of same in male ; bilobed liver, gall
bladder, bile duct ; cardiac and pyloric divisions of stomach-
bursa entiana, intestine, rectum ; pancreas and its duct
rectal gland : spleen (Fig. 326).

5 a. Turn the liver forward and alimentary canal to the left, and
note in addition : coeliac artery, portal vein, testis or ovary,
vas deferens or oviduct.

b. Remove the alimentary canal and its appendages, cut open the

cloaca, remove the dorsal peritoneum on one side. Note: in
male, testes, vasa efferentia, vas deferens, vesicula seminalis,.
sperm sac ; kidney ; cloaca, anus, urinogenital papilla, ab-
dominal pouches ; in female, ovary, oviduct, its internal and
cloacal openings, shell gland ; kidney, its duct and bladder
urinary papilla ; abdominal pores (Figs. 328, 329).

c. Cut open the intestine, wash out, and examine the spiral valve.
6a. Open the pericardium. Note : sinus venosus, auricle,

ventricle, conus, ductus Cuvieri (Figs. 326, 331).

b. Pass a seeker, behind the heart, through the pericardio-

peritoneal canal.

c. Cut open sinus venosus and ductus Cuvieri and find openings-

of : hepatic, posterior cardinal, and anterior cardinal sinuses.
7 a} Skin the throat and dissect away underlying sheet of muscles.:
Note thyroid gland lying between two great strands ot

1 The Piked Dogfish (Acanthias vulgaris), sometimes used for dissection, differs
from the rough hound only in comparatively unimportant points. It is rather
smaller, and more slender, and has a sharp spine in front of each dorsal fin. Its
oronasal grooves are closed, the gall-bladder is more and the thyroid less conspicuous,
the pancreas is variable in size, and both ovaries are present. The fourth and
fifth afferent branchials have a common root. The ophthalmic branch of the fifth
nerve passes ventral to the superior oblique muscle and is therefore probably the-
ophth. profundus, and the maxillary and mandibular branches separate earlier.

2 Parts of the arterial system missed in the dissection owing to lack of blood in
preserved specimens should be identified in injected specimens.

75°

APPENDIX

muscle. Remove thyroid and muscle strands ; note ventral
aorta; trace afferent branchial arteries (Fig. 331).

b. Cut through the jaw and branchial arches on one side, turn
the floor of the mouth outwards, and remove mucous mem-
brane of roof of mouth. Note gill-clefts. Find : gills and
pseudobranch ; efferent branchial and epibranchial
arteries ; dorsal aorta ; carotid arteries, and point of origin
of orbital (“ external carotid ”) arteries ; subclavian and
coeliac arteries (the latter found by tracing back dorsal
aorta) (Fig. 332).

Sa. Cut through the skin round the eye and pull the eyeball
outwards. Note, in orbit : eyeball ; internal, superior, and
external recti muscles ; superior oblique ; optic nerve,
fourth nerve (Fig. 339).

b. Remove the eye by cutting through the six muscles and the

optic nerve, close to the eyeball. Note : four recti muscles,
two obliques ; second, third, and fourth nerves, maxillary,
mandibular, and ophthalmic branches of fifth nerve, hyo-
mandibular, prespiracular palatine, and ophthalmic branches
of seventh (Fig. 340).

c. Cut away root of cranium and neural arches. Note : olfactory

lobes, cerebrum, thalamencephalon, optic lobes, cerebellum,
medulla oblongata, restiform tracts, fourth ventricle, spinal
cord roots of all cranial nerves except sixth, spinal nerves
. .(Fig- 336).

d. blice horizontally through the auditory capsule until nerves

passing through it are exposed ; then slit open the anterior
cardinal sinus and trace distribution of these nerves. Note :
semicircular canals ; roots of ninth and tenth cranial nerves ;
ninth passing to first branchial arch, with branch to hyoid
arch ; branches of tenth to branchial arches 2-5, each with
branchlet to preceding arch, other branches of same nerve
to lateral line, and to heart and viscera (Fig. 321).

€. Cut through nerves and remove the brain. In ventral view,
note : olfactory lobes, cerebrum, optic nerves and chiasma,
infundibulum, lobi inferiores and sacci vasculosi, crura
cerebri, medulla oblongata, restiform tracts ; roots of third
and of fifth to tenth cranial nerves (Fig. 337).

9. Draw the skeleton in the following sections : cranium (side,
dorsal and ventral views) ; jaws and branchial arches ;
vertebral column (side view of portions of trunk and tail
regions, longitudinal section) ; pectoral girdle and fin ;
pelvic girdle and fin (Figs. 319, 320, 322, 323).

In order to obtain these fish in inland places with the viscera
The Cod1 (or intact a special order must be given to a fishmonger.
Haddock or I. It will usually not be possible to observe the

Whiting' animal alive, but a study of the habits of a bony

1 The student who has not already studied the dogfish or some other fish is recom-
mended to read the paragraph which begins at the bottom of p. 420, and also to
-note the following facts : —

In the cod, haddock, and whiting, the body is coated with scales which are thin.

APPENDIX

7 51

fish can conveniently be made by watching a goldfish.
Note : the respiratory movements, and the use of the tail
and fins.

2. In side view, note : the same parts as in the dogfish, with which
make a comparison (p. 750 footnote). Note also : the absence
of eyelids ; and of spiracle ; the gill cover, consisting of the
operculum proper and the expansible branchiostegal mem-
brane ; the double nostril on each side ; the separate anal,
genital, and urinary openings ; the scales (Figs. 348, 347 D).

3 a. Lift the gill cover and observe the gills.

b . Dissect away the skin on one side of the body and note :

myomeres ; lateral line branch of vagus, starting under the
operculum ; the cutaneous branch of the trigeminal (peculiar
to Actinopterygii), starting on the head and branching to
supply sense organs on the fins.

c. Open the mouth, and examine the teeth.

4. Make a flap of the right wall of the abdominal cavity by an

incision passing in tront of the anus, and turn this flap back.
Note : oesophagus, stomach, pyloric caeca, intestine, rectum ;
liver (three lobes), gall bladder, bile duct ; spleen 4 air
bladder ; coeliac artery, portal vein, hepatic veins ; festis or
ovary, vas deterens or oviduct ; ureter. Pass a seeker into
genital duct and into ureter (Fig. 542).

5. Open the air bladder. Note : vascular plexus from which gas

is secreted into bladder ; dorsal aorta. Remove dorsal wall
of air bladder and note kidneys.

flat plates of bone, completely covered by skin and overlapping backwards ; it is
shaped for cleaving the water. The mouth is at the front end ; in life it is
continually opening and taking in gulps of water, which passes into the pharynx
and outward on the sides of the head through five gill-clefts that open under a gill-
cover or operculum on each side. Above the mouth lie the nostrils, which have no
internal openings. A shallow ventral depression in which lie the anal, genital, and
urinary openings is placed midway underneath. The widest part of the body
contains the body cavity, which reaches some way behind the vent. The narrower
hinder region or tail contains only muscle and backbone, and ends in two equal
flukes, which are used in propelling the fish (p. 423). The two pairs of limbs
or fins, used in steering, balancing, and backing, are quite unlike the corresponding
members of a frog or a man, and the pelvic or true hinder pair lies forward under
the throat. They and the dorsal and ventral fins by which the body is kept upright
are strengthened by rays. There is no neck. The body is stiffened by the back-
bone, composed of a row of bony rings or vertebrae, which runs along the upper
side of its whole length except in the head, where it is continued by the skull. In
the middle region ribs arch outwards from the backbone in the body-wall. There
is no breastbone. As in all Vertebrata, the central nervous system lies in the
hollow of the skull and backbone, the body cavity below the latter, containing the
stomach, coiled intestine, and other viscera, and the heart in a part of the body
cavity known as the pericardial cavity, which here stands in front of the rest and is
cut off from it. From the heart, a ventral arterial system, consisting of a median
ventral aorta and an afferent branchial artery in each of the “ branchial arches”
which separate the gill-clefts, conveys blood to the gills, which stand upon the

arches, whence it passes, oxygenated, to the body by means of a dorsal arterial

system. In returning to the heart, most of it passes either through a hepatic portal
system in the liver or through a renal portal system in the kidneys. The latter are
dark brown strips of tissue lying under the backbone above the body cavity, from
which they are separated by a large, closed air-bladder. The genital ducts are

continuous with the gonads in both sexes, and have no connection with the

kidneys.

APPENDIX

6, Cut through girdles and note : heart in pericardium, its parts
(Fig. 349).

t'lG. S42. — A semi-diagrammatic view of the contents of the abdominal
cavity of a male cod which has been opened on the right-hand side.

z.b., Air bladder; an., anus; bl., bladder; d., duodenum; g ., genital opening,
g.b., gall-bladder; ini., ileum; kd., kidney duct; lr., liver; oes., oesophagus,
Py.c., pyloric caeca ; rtn., rectum ; st stomach ; t., testis ; ur., urinary opening.

7 a. Dissect away muscles, etc., in front of the. heart, and trace ;
ventral aorta and afferent branchial arteries (Fig. 349).
b. Cut away the floor of the mouth, and on its roof dissect out
the efferent branchial system (Fig. 350).

Fig. ^43. — The brain and its nerves in a cod.

Corpora striata, two large masses or ganglia of grey matter which lie at the
base of the cerebrum of vertebrata and are here exposed because the roof of the
cerebrum, which in the cod is thin and non-nervous, has been torn away ; cb.,
cerebellum ; l.i., right lobus inferior ; m.o., medulla oblongata ; ol/.st., stalk of
olfactory lobe ; op.L, optic lobes ; II.-X., cranial nerves ; V .md., V.mx., V.op.,
mandibular, maxillary, and ophthalmic branches of the fifth nerve; VII. hd.,
VII. hm., VII. m., VI I. op., VII.p.sp., Vll.pal.b., hyoidean, hyomandibuiar,
mandibular, palatobuccal, and ophthalmic branches of seventh nerve ; X.c.,
cutaneous branch of the vagus nerve, from which the lateral line is innervated
The cutaneous branch of the fifth nerve is noS shown.

APPENDIX

753

8 a. Cut away the roof of the cranium and expose the brain.
Note its regions and trace out the nerves. The ophthalmic
is the superficial one (Fig. 543).

b. Cut through the auditory capsule. Note labyrinth, with large-
otolith in vestibule.

9 a. In skull, note the following bones : in cranium, basioccipitaC
exoccipitals, supraoccipital, parietals, alisphenoids, frontals,.
parasphenoid ; in auditory capsule of one side, epiotic,
pterotic, opisthotic, prootic, sphenotic ; in either orbit
lacrymal, suborbitals ; in nasal region, nasals, vomer,,
mesethmoid, parethmoids ; in either upper jaw palatine,.

Fig. 544.— The skull of a cod, from the left side. — From Reynolds.

1 and 2, Lacrymal ; 3, median ethmoid ; 4, frontal ; 5, supraoccipital ; 6, pterotic
7, orbit ; immediately below is the parasphenoid ; 8, pterygoid ; 9, suborbital
10, bones of orbital ring; 11, maxilla; 12, premaxilla; 13, hyomandibular
14, symplectic ; 15, quadrate; 16, metapterygoid; 17, opercular; 18, sub-
opercular ; 19, preopercular ; 20, interopercular ; 21, articular ; 22, dentary.

pterygoid, mesopterygoid, metapterygoid, quadrate, pre-
maxilla, maxilla ; in either lower jaw, articular, angular,
dentary ; in either hyoid arch, hyomandibular, symplectic,.
epihyal, ceratohyal, hypohyals, urohyal, brachiostegal rays ;
in either operculum, opercular, preopercular, subopercular,
interopercular (Fig. 544).

b. Note the parts of the vertebrae.

c. In the shoulder girdle and fore-fin skeleton, note : post-

temporal, supracleithrum, cleithrum (“ clavicle”), post-cleith-
rum, scapula, coracoid, brachial ossicles, fin rays (Fig. 545),
a. In the pelvic girdle note that each half consists of a single
bone, in which iliac, ischial, and pubic regions are not
distinguishable.

48

754

appendix

Lizards are found in various parts of England, but it will usually be
most convenient to obtain specimens from a dealer.

The Lizard- i. The animal should be kept under observation

alive. Kill with chloroform.

2. Note : head, neck, trunk, tail, limbs, mouth, nostril, eye,
with three lids ; shallow depression for ear ; vent ; scales
(Fig. 359 A).

3 a. Hand and foot. Count the digits.
b. Open the mouth. Note teeth, tongue, glottis, nares.

Fig. 545.— The right half of the pectoral girdle and right pectoral
fin of a cod. — From Reynolds.

1, Tost-temporal ; 2, supracleithrura ; 3, cleithrum; 4, coracoid; 5, scapula;

6, post-cleithrum ; 7, brachial ossicles ; 8, dermal fin rays.

4. Pin the animal on its back. Cut through the abdominal body-

wall to one side of middle line (note that skin adheres to
muscles) and through the pectoral girdle, and turn back.
Note : anterior abdominal vein on body-wall ; heart in
pericardium, aortic arches, inferior vena cava ; lungs ;
liver and gall bladder ; stomach, duodenum, ileum, rectum,
cloaca, bladder ; pancreas ; fat bodies in hinder part of
body-wall (Fig. 34b).

5. Cut through the liver on the left side of the animal. Turn the

viscera to left, and spread them out. Note : course of
alimentary canal ; and principal blood vessels (Fig. 548) $

APPENDIX

755

testes or ovaries, epididymides, vasa deferentia, or oviducts,
kidneys, Cut open cloaca and note openings of rectum,
bladder, ureters, and generative ducts (Fig. 547).

6. Open the pericardium and note parts of heart and roots of
great vessels (Fig. 548).

Fig. 546.— The viscera of the Sand Lizard ( Lacerta agilis), in
their natural relations. — From Weidersheim.

31.. Bladder ; Ct.t inferior vena cava ; ED., rectum ; GB ., gall bladder ;
H., heart ; Lg., Lg'., lungs ; M., stomach ; MD., small intestine
Of., oesophagus ; Pn., pancreas ; Tr., trachea.

00

APPENDIX

•56

. Dissect up throat and trace trachea, gullet, jugulars.

Break away the roof of the skull and observe dorsal \ie^ o-
brain. Remove brain and observe ventral view. Note that
it resembles that of the frog in general features, but has
larger cerebral hemispheres and twelve nerves.

9. In the skull of the common lizard the supratemporal fossa is
bridged by bones. For this reason, and because of the small
size of the animal, it is convenient to study instead of its
skeleton that of some other and larger kind (e.g. Uromastix).
In the skull (Fig. 549)' note the following bones : in the

Fig. 547,— The urinary and genital organs of a lizard ( Lactrta ).

A, Male ; B, female.

hi Bladder ; cl., cloaca ; e pd., epididymis ; i.o.d., internal opening of the oviduct ;
’ int., ileum; k., kidney; od ., oviduct; od'., external opening of oviduct ; ov.,
ovary; /«., penes; r.p., retractor muscles of the penes; rm., rectum; t
testes; we-, urinogenital opening; ur., ureter; ur '., opening of ureter; v.d.,
vas deferens.

cranium, supraoccipital, exoccipitals, basioccipital ; pari-
etal,1 epipterygoids, basisphenoid ; frontal ; 1 parasphenoid ;
at the sides of the cranium, squamosals, supratemporals,
postfrontals, prefrontals, lacrymals ; in either auditory
capsule, prootic ; in nasal region, vomer 1 and nasals ; in
either upper jaw, premaxilla,1 maxilla, jugal, palatine,
transpalatine, pterygoid, . quadrate ; in either lower jaw.
dentary, Meckel’s cartilage, angular, splenial, supra-
angular, coronoid. Examine also the other parts of the
skeleton (Fig. 550 for shoulder girdle).

1 Two fused.

APPENDIX

757

r. The student snouid not fail to make himself acquainted with
the many interesting facts which may be learnt from
The Pigeon, watching pigeons, concerning, for instance, the nesting
care of the young, and their feeding ; the use of the
oil gland ; moulting ; the use of the wings and of the tail

Fig. 54S. — The principal arteries of a lizard.

a.w., Anterior mesenteric; c., coeliac; c.a., carotid arch; c.c common
carotid; c.m., coeliaco-mesenteric ; cd., caudal; d.ao., dorsal aorta;
d.c. ductus caroticus ; f. femoral or external iliac ; l.ov., left ovarian :
/>., pulmonary; p.m., posterior mesenteric; r., renal; r.h., right
hepatic ; the left hepatic is a branch of the coe,:ac ; r.ov., right
ovarian ; scl. , subclavian ; set., sciatic or internal iliac ; sy.a., systemic
arch ; tr.L, tracheo-lingual.

in flight and gliding, steering and alighting ; courting ;
challenging, and fighting with beak and wing.

3. Of external features, note the head, neck, trunk, very short
tail ; beak, cere, nostrils ; eyes, with three eyelids ; ear

758

APPENDIX

openings ; vent ; wings and legs ; reptile-like scales on
latter (Figs. 369, 370).

3 a. Pluck the bird. In so doing note arrangement of feathers,
especially on wings (Figs. 369, 370, 375).

b. Note parts of wing, alar membranes, bastard wing (Fig. 370).

c . Examine the various kinds of feathers (Fig. 372).

d. Note pterylse and apteria (Figs. 369, 370).

4 a. Skin the ventral surface of thorax, abdomen, and neck. Note

A ' 8

Fig. 549. — The skull of the lizard Uromastix.

A , E orsal view ; B, ventral view

al.sp'., Cartilage representing alisphenoid ; b.oc., basioccipital ; b.sp., basisphenoSd ;
col., columella auris ; epipi., epipterygoid ; ex.oc., exoccipital ; fr., frontal;
ju., jugal ; mx., maxilla ; n., nasal ; nas' ., region from which cartilaginous
nasal capsule has been removed ; oc., occipital condyle ; orb., orbit ; par.,
parietal ; pi., palatine ; pm., premaxilla ; prf., prefrontal and lachrymal, fused;
pro., prootic; pt., pterygoid; plf., postfrontal and postorbital, fused;
q., quadrate ; rs., rostrum ; s.oc., supraoccipital ; s.t.f., supratemporal foramen ;
sq., squamosal ; sut., supratemporal bone (very small) ; tpt., transpalatine ;
v., vomer (“ prevomer ”)

on the underpart of the neck the crop. >• Separate the large
superficial muscle of the breast (pectoralis major) of one side
from the keel of the sternum and from the entire length of
the clavicle. Turn the muscle forwards, taking care not to
injure the blood vessels of the armpits. Observe close to
these vessels the axillary air sacs (branches of the inter-
clavicular). Note the deeper muscle of the breast (pectoralis
minor) still attached. Cut through its origin, turn it for-
wards and make out the insertion of both breast muscles
and their mode of action. Dissect away the pectoral muscles
of the other side in the same way ; open the abdomen and .

appendix

759

raise the hind end of the sternum, cut through the attach -
ment of the sternum to ribs and coracoid on each side, and
remove the sternum. Note : gullet, crop ; thymus ;
trachea, muscles of syrinx ; heart in pericardium, roots of
arteries ; liver, gizzard, duodenum, pancreas, ileum ;
thoracic and abdominal air sacs (Fig. 385).
b. Displace viscera and examine them. Note, in addition to

of a rib.

The dotted regions are cartilage.

parts already seen,* proventriculus, rectum and its caeca,
cloaca; pancreatic and bile ducts; spleen, lungs, . portal,
epigastric (anterior abdominal) and coccygeomesenteric veins.

5. Remove the alimentary canal. Note : kidneys, ureters ; testes

or ovary, vasa deferentia or oviducts (right oviduct small)
(Figs. 386, 387) ; veins (Fig. 390), arteries (Fig. 3S9).

6. Remove the pericardium. Note great vessels entering and

leaving heart (Fig. 389).

7#, Displace the gullet and windpipe and trace the carotid arteries
and jugular veins along the neck.

760

APPENDIX

7 b. Trace the brachial and pectoral arteries and veins to the wings.

8. Sking the top of the head and very carefully remove the root of

the skull. Note the parts of the brain in dorsal, and after
its removal in ventral view (Fig. 391).

9. The skeleton of the Common Fowl is often studied instead of

that of the pigeon. Its parts should be examined with the
aid of the figures and description on pp. 497-503.

In buying rabbits for dissection, avoid those that are likely to be
very fat.

1. Wild rabbits can be watched if the observer will
The Rabbit. keep still. Feeding and breeding habits are better

observed in tame ones.

2. Note : head, neck, trunk, tail, limbs, the proportions of the
parts of the latter ; nostrils, lips, teeth ; eyes, their lids ;
whiskers ; ears ; anus ; urinogenital opening.

2)U. Skin the face. Note: masseter muscle, facial nerve, parotid gland.

b. In the orbit, find lacrymal and infraorbital glands.

c. Cut through muscles at sides of mouth and open it widely.

Note : teeth (count), hard palate, soft palate, posterior nares,
pharynx, beginning of oesophagus, glottis, epiglottis, tongue.
Slit open soft palate and find Eustachian tubes.

4 a. Fasten the animal on its back. Skin the abdomen and turn
flaps of skin outward. Make a median and a transverse cut
through abdominal muscles and turn flaps outwards. Note :
xiphisternum ; falciform ligament ; liver ; stomach ; small
intestine, caecum, colon, rectum ; urinary bladder (Fig. 41 1).

b. Turn up the liver and spread out the duodenum. Note :
liver, gall bladder, bile duct ; portal vein, hepatic artery ;
oesophagus, cardiac and pyloric regions of stomach, duo-
denum ; pancreas, its duct (Fig. 412) ; spleen.

,c. Turn the stomach and intestine to the animal's right. Note :
kidney ; adrenal body ; dorsal aorta, coeliac, hepatic, lieno-
gastric, anterior mesenteric, left renal, and posterior mesen-
teric arteries ; inferior vena cava, left renal vein ; left vagus
on oesophagus, solar plexus, left splanchnic nerve (Fig. 426).

5 a. Doubly ligature the portal vein and cut between the ligatures,
ligature oesophagus and cut above ligature, ligature and cut
rectum, cut through mesentery, remove alimentary canal,
and spread out. Note : oesophagus, stomach, pyloric
sphincter, duodenum, ileum, sacculus rotundus, caecum,
vermiform appendix, colon, rectum (Fig. 413).

b. In the abdomen, note : muscular portion and central tendon
of diaphragm ; oesophagus ; kidneys ; suprarenal bodies ;
ureters, bladder ; dorsal aorta, coeliac, anterior mesenteric,
renal, genital, posterior mesenteric, and common iliac
arteries ; inferior vena cava, renal, genital, and external
and internal iliac veins (Figs. 416, 420) ; in male, testis
(pulled out of scrotal sac), epididymis, vas deferens, uterus
masculinus, spermatic cord (Fig. 416) ; in female, ovaries,
oviducts, internal openings of same, uteri, vagina.

APPENDIX

761

Cut away the ventral part of the pelvic girdle. Note : in male,
rectum, bladder, ureter, vas deferens, uterus masculinus,
prostrate, perineal gland (Fig. 4 17> A) ; in female, rectum,
bladder, ureter, vagina, vestibule, perineal gland (Fig. 417, B).
&a. Cut through the ribs and remove the sternum. Note . ribs,
intercostal muscles ; thymus gland ; heart ; lungs , phrenic
nerves: diaphragm; 'pleural cavities (Fig. 414, thymus
removed).

b. Remove the thymus and pericardium and turn the heart to
animal's right. Note : left auricle, left ventricle, parts of
right auricle and ventricle ; pulmonary aiteries , arch of
aorta ; innominate, right subclavian, right common carotid,
left common carotid and left subclavian arteries ; dorsal
aorta, passing through diaphragm ; left superior vena cava,
left subclavian, external jugular, and pulmonary veins ,
interior vena cava, oesophagus (Fig. 4*4? lover part).

After doing No. 7. cut through great vessels at some distance
from the heart and remove the latter. N ote in dorsal and ventral
views of it the chambers and vessels mentioned above.1 _
d. Cut open the right side of the heart. Note thin wall of auricle,
thick wall of ventricle, column* carnete, flaps of tricuspid
valve, chordae tendinese and papallary muscles, semilunai
valves, openings of veins (Fig. 418).

7. Remove the skin of the neck and underlying muscles. Note
and trace : posterior cornu of hyoid ; tendon of mandibular
muscle ; larynx, trachea ; thyroid gland ; . oesophagus ,
common, external, and internal carotid arteiies externa
and internal jugular, anterior and posterior facial veins ,
submaxillary and parotid glands ; main vagus, superior
and recurrent laryngeal, depressor, hypoglossal, and.
cervical sympathetic nerves ; vagus and superior cervical

sympathetic ganglia (Fig. 414). . .

S. Remove the brain. Note : in dorsal view, olfactory lobes,
cerebral hemisphere, cerebellum, medulla oblongata, fourth
ventricle. Pull outwards, one cerebral hemisphere, and note :
corpus callosum, optic thalami, corpora quadrigemma (big.
423). In ventral view, olfactory, temporal, and pyriform
lobes ; infundibulum ; pituitary body ; corpus albicans ,
crura cerebri : pons Yarolii ; medulla oblongata ; roots o
second, third, and fifth to twelfth cranial nerves (Fig. 424)/
q. Examine the several parts of the skeleton, taking the following
vertebrae as examples of their series : atlas, axis, third
cervical, fourth thoracic, second lumbar, sacrum (Figs. 397
409). 3

1 The heart of a sheep is often dissected instead of that of the rabbit. The chief
difference between the two is that, like those of man the two superior venae cavae
{innominate veins) of the sheep unite before entering the auncle.

X 2 The brain of the sheep is often examined instead of that of the rabbit its
features will readilv be followed from the description of the rabbit s brain, but it will

£ noted that the cerebral hemispheres are much better P“«d«d. ”‘h

3 The skull of the dog is often studied m preference to that of the rabbit, bee the

footnote on p. 528 (Figs. 446 and 447).

762

APPENDIX

so a. Examine under the microscope a drop of blood. Compare
the red corpuscles (i) with those of the frog, which you have
already seen (Fig. 8i) (ii) with those in a drop of your own
blood (Fig. 82).

b. If possible, sections prepared by an expert from the principal
organs (liver, lungs, pancreas, salivary glands, small in-
testine, testis, etc.) of a mammal should be examined, with
the aid of some work on mammalian histology.

Fig. 55 *• — A diagrammatic representation of the spermatogenesis

of the Rat. — After Schafer.

Each of the numbered sections of the diagram represents a portion of the circum-
ference of a seminiferous tubule at a certain stage of the process. In (r) the cells
(spermatids) which result from the two successive maturation divisions of the
spermatocytes, and eventually become spermatozoa, are seen in their earliest
condition. In (2) they have become attached in groups to supporting cells
(cells of Sertoli). In (3) to (8) they are becoming spermatozoa, of which their
nuclei constitute the heads. In (1) again, they are ready to be set free.

®» anJ~ a. >. Lining epithelium cells of the tubules ; a are “ spermatogonia,” which by
division (seen in 6) throw off spermatocytes; a' are cells of Sertoli, which
support the spermatids ; b, spermatocytes. These undergo the two maturation
divisions (indicated in 5) whose ultimate products are c, the spermatids. The
latter, in the process of development into spermatozoa, which they undergo
after attachment to cells of Sertoli, throw off s', portions of their cytoplasm
which disintegrate (s).

I N D E X

Numbers in heavy type refer to pages with illustrations of the
structure in question.

Separate references are not given to each mention of an organ
in the section on practical work.

Abdomen of cockroach, 325.
of crayfish, 288.
of frog, 33.

transverse section of, 35.
of rabbit, 521.

A.bdominal or peritoneal cav-
ity, 521.

limbs of crayfish, 295.
pores, 420.

Abiogenesis, 725.

Abomasum, 577.

Absorption, 9, 11, 17.
Acalephse, 234, 679.

Acarina, 318, 681.

Acetabular facet, 432.
Acetabulum, 48 (legend),
of frog, 47.
of pigeon, 503.
of rabbit, 535, 536.
Aciculum, 280.

Acini, 1 1 2.

Acipenser, 466.

Acquired characters, 702, 705.
Acromion, 45.

Actiniaria, 233.

Actinopterygii, 463, 466, 467,
468, 684.

Action, reflex and voluntary,
95> 96-

Activity, direction of. See
Purposiveness.
Adambulacral spines, 392.

ossicles, 394.

Adaptation, 20.

Adaptive radiation, 717, 719.
Adenoid tissue, 127.

Adrenal bodies, 22, 63, 459.
cortex of, 63, 459.
medulla of, 63, 64, 459.
of dogfish, 459.

Adrenal bodies of frog, 63, 64.
of rabbit, 546, 548.
glands. See Adrenal bodies.
Adrenalin, 22, 64.

Adsorption, 195.

Adult, 14.

Aerobic organisms, 4, 367.
Afferent, or sensory, fibres, 93,
nerves, 92, 96.
root, 86.

Afterbirth, 665.

Aftershaft, 494.

Agglutinins, 124.

Air bladder of Teleostei, 468.
Air capillaries of the pigeon,
509-

sacs of the pigeon, 508, 509.
Alae cordis, 302.

Alary muscles, 332.

Albumin, 9.

Alcohol, 3.

Algae, 713.

Alimentary canal. See Canal.
Alive, 1.

All-or-none phenomena, 94.
Allantois, 474, 476, 649, 651.
early developed, 669.
of bird, 649, 657.
of mammal, 654, 661, 662.
origin of, 644.

present in reptiles, 476, 685.
Allelomorphs, 699.

Alligators, 487.

Allolobophora, prostomium of,
257 fn.

Alternation of generations, 230,
248.

in Aphis, 340.
in liver fluke, 248.
in malaria parasite, 189, 230.
76 }

7$4

INDEX

Alternation of generations in
Monocystis, 159, 230.
in Obelia, 230.
in tapeworms, 253, 254.
Alveolar layer in Vorticella,

I74-

Ambulacra of Echinoidea, 403.
Ambulacral groove of starfish,
389-

ossicles, 394.
ridge, 392.
spines, 392.

Ametabola, 336, 681.
Amino-acids, 9, 156.

Amitotic division, 128.
Ammoccetes, 463.

Amnion, 474, 476, 649, 659.
early developed, 661, 669.
false, 649, 660.
of bird, 649.

of mammal, 659, 660, 662,
665.

of man, 661.

origin of, in embryo, 644,
649-

present in reptiles, 476, 685.
true, 649.

Amoeba, 123, 140, 678, 738.
and the cell theory, 149.
proteus, 140, 141, 142, 143,
738.

binary fission in, 145, 146.
depression in, 145.
encystment of, 145.
excretion in, 145.
immortality of, 150.
irritability, automatism,
and conductivity in,
144.

movements of, 141.
multiple fission in, 146,
147-

nutrition of, 143.
reproduction of, 146, 594.
respiration of, 145.
spore formation in, 146,
147.

Amoebae, multinucleate, 147.
Amoeboid movement, 123.
Amoebulae, 159.

Amphibia, 474, 684.

Amphioxus. See Lancelet.
Ampullae of frog’s ear, 101.
neuromast, 458.
of tube feet, 396.

Amylase, 61.

Anabolism, 12.

Anaerobic organisms, 4, 367.
Anal cerci, of cockroach, 325.

cirri, of Nereis, 280.
Analogous organs, 691.
Anaphase, 129, 134.
Anapophysis, 526.

Anatomy, definition of, 30.
Ancylostoma duodenale, 360,
361.

Anemones, sea-, 233, 234.
Anguillula, 627.

aceti, 360, 360.

Animal pole, 605.

Animals, difference between
plants and, 23, 24, 29.
study of, 1, 733.

Anisogamy, 154.

Annelida, 286, 680.

Annuli of leech, 282.

Annulus of Yieussens, 92.
Anodonta, 682. See also
Mussel, swan.

Anopheles, 192, 195, 196, 344,
345, 346.

Ansa subclavia, 92.

Ant-eaters, 571.

Antebrachium, 34.

Antennae, first. See Antennule.
of cockroach, 323.
of crayfish, 292, 293, 296, 314.
of crustaceans, 316.
of housefly, 346.
of Lepidoptera, 351.
Antennule of crayfish, 292, 31 1.
Anthozoa, 679.

Anthropoidea, 686.

Antibodies, 124.

Antitoxins, 124, 367.

Antlers, 577, 577.

Ants, 337, 343.

Anura, 474, 685,

Anus, 58.

absent in liver fluke, 241.

in Platyhelminthes, 239.
of Ascaris, 353.

INDEX

7 65

Anus of cockroach, 326-
of crayfish, 288.
of earthworm, 256, 265.
of frog, 58.
of gastropoda, 3S5.
of lancelet, 403.
of pigeon, 50(3.
of rabbit, 518.
of starfish, 390.
of swan mussel. 376.
of Voriicella, 175
temporary, of Paramecium,
165-

Aorta, anterior and posterior,
of swan mussel, 382.
branchial, of Amphioxus,
4i5

dorsal, 71, 633, 654.
of Amphioxus, 415.
of chick, 654, 655.
of dogfish, 444, 446-
of frog, 71.
of pigeon, 511.
of rabbit, 554, 556-
ventral, 415, 632.
of Amphioxus, 415.
of dogfish, 444-
of vertebrate embryo, 632.
of frog, 66.
of tadpole, 631, 632.

Aortic arches, 632.

of birds, 482, 632, 654.
of dogfish, 444.
of frog, 70, 71.
of mammals, 482, 632.
of newts, 475, 478.
of pigeon, 512.
of rabbit, 554, 556-
of reptiles, 481 .
of tadpoles, 631, 633.
of vertebrate embryos,

632.

Aphaniptera, 34S, 68 1.
Aphetohy oidea , 471, 683,
Aphis, 340, 341.

Aponeurosis, 50.

Apopyles, 204.

Apparatus for practical work,

729- . J ,

Appendages (paired), 01

Arthropoda, 316, 680.

Appendages of Arachnida.,

31 7-

of Araneida, 318.
of cockroach, 323.
of crayfish, 289.
of insects, 317.
mouth, of Acarina, 3J8-
of cockroach, 326.
of crayfish, 293.
of insects, 338, 342, 343*
344, 345, 346, 347- 348'
of vertebrate. See Limbs.
Apteria, 492.

Apterygota, 336, 681.
Aquaeductus cerebri, of frog,.
87.

of rabbit, 563.
vestibuli, of dogfish, 426.
Arachnida, 317, 318, 681.
Araneida, 318, 681.

Arborisation, terminal, of axon,

H5-

Arc, the reflex, 96.

Arcades of skull, 47^- 479>
481, 482.

Arch, pectoral. See Girdle,,

pectoral.

pelvic. See Girdle, pelvic.
Archaeopteryx, 515, 5l6- 695.
Archenteron, 607, 609, 610.
Arches, aortic. See Aortic

arches.

arterial, of chick, 654.
of frog, 70, 71.
carotid, 70.
pulmocutaneous, 70.
systemic, 70.

' of tadpole, 632, 633.
branchial, of dogfish, 429*
43°- 436.

haemal, of dogfish, 420.
hyoid, of dogfish, 429.
mandibular, of dogfish, 429.

of tadpole, 632.
visceral, 436.

of dogfish’s skeleton, 429.
of tadpole, 628.

Area opaca and area pellucida,
641.

vasculosa, 653.

Arenicola, 286, 680.

766

INDEX

Arm of frog, 34, 535, 536.
of man, 536, 537, 590.
of Primates, 584.

Armadillos, 571.

Arms of starfish, 389.

Arolium, 325.

Arterial blood, 76.

system. See Arteries.
Arteries, 69.
allantoic, 654.
of crayfish, 302.
of dogfish, 442, 444.
of frog, 70.
of lizard, 757.
of pigeon, 511, 512.
of rabbit, 556.
of tadpole, 631, 632.
of vertebrate embryo, 632.
vitelline, 654.

Arterioles, 69.

Artery —

afferent branchial, 444, 465.
antennary, of crayfish, 302.
anterior mesenteric, 71, 445,
556.

brachial, 51 1.
caudal, 445, 556.
c celiac, 71, 445, 556.
coeliaco-mesenteric, 71.
common carotid, 70, 445,
512, 556.
cutaneous, 70.

dorsal abdominal, of cray-
fish, 304.

efferent branchial, 445.
epibranchial, 445.
external carotid, 70, 446, 556.
gastric, of crayfish, 304.
genital, 446, 556. See also
Ovarian, spermatic,
hepatic, 71, 446, 556.

of crayfish, 304.
iliac, 71, 556. '
innominate, 512, 556.
internal carotid, 70, 446,

556.

lienogastric, 446, 356.
lingual, 70.
occipitovertebral, 71.
oesophageal, 71.
ophthalmic, of crayfish, 302.

Artery —
orbital, 70.
ovarian, 71.
pectoral, 512.

posterior mesenteric, 71,
446, 556.

pulmonary, 70, 481, 512,

554- "

renal, 71, 446, 556.
spermatic, 71.
spiracular, 446.
stapedial. See orbital,
sternal, of crayfish, 304.
subclavian, 71, 446, 556.
suprabranchial, 445, 469,

632, 633.

ventral abdominal, of cray-
fish, 304.

thoracic, of crayfish, 304.
See also Aorta, Aorta arches.

Arthrobranchiae, 307.

Arthropoda, 316, 368, 680, 681.

Artiodactyla, 576, 578, 686.

A scans, 680, 745.

alimentary canal of, 356.
excretory organs of, 354, 357.
generative organs of, 357.
infective larvae of, 358.
lumbricoides, body-wall of,
354-.

lumbricoides and A . megalo-
cephala, 352, 353, 354,
355. 358. 359.
muscle fibres of, 356.
nervous system of, 355, 356,
357. '

Asexual reproduction. See
Reproduction, asexual.

Assimilation, 12, 17, 126, 723.

Astacus, 680. See also Cray-
fish.

torrentium and fluviatilis,
287.

Aster, 129.

Asterias, 682.

rubens, 389, 390, 391, 395,
399.

Asteroidea, 400, 403.

Atlas, 524.

Atriopore, 405.

Atrium of Amphioxus, 406.

INDEX

767

Atrium = auricle, 65. See also
Heart.

Auditory capsules, 39, 44.
organs. See Ears.

Aurelia, 236, 237, 679.
life-history of, 237.

Auricle. See Heart.

Auriculo-ventricular valves of
frog’s heart, 66.

Automatism, 8, 17, 126.
in Amoeba, 144.
in leucocytes, 125.
in Paramecium, 165.
in plants, 24.
in protoplasm, 126.

Autonomic, system, 92, 93.

See also Sympathetic
system.

Autotomy, in crayfish, 298.

Aves, 460, 685.

Axial organ of starfish, 394,

395, 399, 4GI-

Axis, cylinder, 94, 115.

of the body, factors in ovum
determining, 669.
vertebra, 524.

Axon, 1 15.

Backbone, 36, 421.
of dogfish, 424.
of frog, 36.
of pigeon, 499.
of rabbit, 522.
of snakes, 484.

See also Vertebrae.

Bacteria, 713, 713.
and flies, 348.
role of, in digestion, 546.
in nitrification, 30.
in putrefaction, 713.

Baer, von, law of, 668, 692.

Balcenoptera, fore-limb of, 571.

Balance of nature, 29, 714.

Balantidium, 678, 739.

coli and B. entozoon, 195,
197, 198, 739-

Bands, peripharyngeal, of
Amphioxus, 412.
of Ammocoetes, 463.

Barbicels, 494.

Barbs of feather, 494.

Barbules, 494.

Bars, gill, of Amphioxus, 406,
410, 412.

tongue, of Amphioxus,
410.

Basal granule, 163, 183.

Basipodite, 289.

Basipterygium, 432.

Bastard wing, 495.

Bat, wing of, 582, 691.

Bats, 593.

Batteries of nematocysts,
210.

Bed bug, 338, 339.

Bee, head of, 338.

Bees, 337, 343, 343.

Beetles, 335, 337, 338, 342.

Behaviour, 166.

of Amoeba, 144, 166.
of Hydra, 215, 218.
of Paramecium, 165, 167.

Bile, 61.

Bile duct, 59.
of dogfish, 436.
of frog, 59-
of man, 390.
of pigeon, 506.
of rabbit, 544.
reflex opening of, 96, 97.

Bilharzia, 249.

Binary fission —

in Amoeba, 145, 146.
in Balantidium, 196.
in Entamoeba, 179, 181.
in Euglena, 155.
in Hydra, 222.
in Paramecium, 166, 171.
in Polytoma, 152.
in Trypanosoma, 186.
in V orticella, 175.

Biogenesis, 725.

Biology, definition of, 1, 30.
problem of, 721.

Bipinnaria larva, 402.

Bird, wing of, 582.

Birds. See Aves.
and reptiles, 515.

Bladder, gall-, 59.

absent in pigeon, 506.

in Perissodactyla, 578.
of dogfish, 436.

768

INDEX

Bladder, gall, of frog, 59.
of rabbit, 544.
salivary, of cockroach, 328.
urinary, 59.

absent in dogfish, 436.
of frog, 59, 79, 125.
of rabbit, 348, 664.
worm, Cvsticercus or, 252,
254-

diagrams of, 253.

Blastocoele —

of Amphioxus, 606.
of annelid larva, 281.
of chick, 639.
of frog, 616.

Blastocyst. See Blastodermic
vesicle.

Blastoderm, 638.

Blastodermic vesicle, 660.

Blastomeres, 137, 279.

of Amphibia, potentialities
of, 670, 671.
of Amphioxus, bob.
of frog, 137, 616.
of Hydra, 220.

Blastopore, 607, 609, 617, 618,
619, 620, 643.
of Amphioxus, 607.
of chick, 642, 643.
of frog, 617, 618, 619, 620.
of mammals, 657.

B-lastostyles, 229.

Blastula, 606, 617.
of Amphioxus, 606.
of crayfish, 668.
of frog, 616, 617.
of Hydra , 220, 666.
of liver fluke, 248.
of Obelia, 228.
of starfish, earthworm, and
swan mussel, 666, 667.

Blepharoplast, 183.

Blind spot, 313.

Blood. 21, 69, 73, 76, 122.
arterial and venous, 76.
functions of the, 73.
islands, 653.
of crayfish, 306.
of dogfish, 448.
of earthworm, 269.
of frog, 69, 73, 76, 122, 130.

Blood of lancelet, 415.
of leech, 285.
of man, 1 23, 130.
of pigeon, 310.
of rabbit, 537.
of swan mussel, 382.
oxygenation of, in frog, 76,
in crayfish, 303, 306
in dogfish, 444.

See also Respiration,
temperature of, 74, 460, 310.,
557> 066.

Blood corpuscles. See Blood.

vessels. See Vascular system.
Blow-flies, 343, 348, 708.

I Bodies, organised and un-
organised, 724.

Body. See Pineal body, Re-
productive body, etc.
cavities, 273. See also Peri-
visceral cavity,
cavity, primary. See Haemo-
coele.

secondary. See Coelom,
-wall, 35.

of earthworm, 260.
of Hydra, 209.

Bojanus, organs of, 380.

Bone, 121.

cells or corpuscles, 122.
fine structure of, 19, 22,
121.

growth of, effect of pituitary-
hormone on, 673.
marrow, 122.

Bone or bones —

alisphenoid, 300, 328, 590,

753-

angular, 502, 753.
angulosplenial, 44.
articular, 302, 733.
astragalus, 49, 539.
basioccipital, 477, 500, 52S„

59° > 753-

basisphe'noid, 500, 52S, 590.
basitemporal, 300.
calcaneum, 49, 539.
calcar, 49.

cannon. See Cannon bones,
capitate, 537.
carpal, 48.

INDEX

769

Bone or bones —
central, 48 fn., 536.
ceratohyal, 753.
clavicle, 45, 484, 486, 502,
535-

cleithrum, 753, 754.
coccyx, 583.
coffin. See Coffin bone,
columella auris, 42, 502.
coracoid, 45, 484, 502, 535,

567-

coronoid, 756.
cuboid, 539.

cuneiform. See triquetral,
dentary, 44, 502, 753.
distal carpal, 48, 502, 537.

tarsal, 491, 503.
ectocuneiform, 539.
epihyal, 753.
epiotic, 753.
epipterygoid, 483, 756.
ethmoid, 590.

exoccipital, 41, 44, 477, 483,
528, 590, 753.
femur, 49, 503, 538.
fibula, 49, 503, 539, 590.
fibular, of ankle, 49, 539.

See also calcaneum.
frontal, 477, 478, 482, 483,
501, 528, 753.
frontoparietal, 41, 478.
greater multangular, 537.
hamate, 538.
humerus, 47, 502, 536.
hyoid, 44, 45, 502, 533, 535.
hyomandibular, 753.
hypohyal, 753.
ilium, 47, 503, 535.
innominate, 494.
interclavicle, 484, 486.
intermediate (intermedium)
of carpus, 48, 536.
of tarsus, 48, 539.
interopercular, 753.
interparietal, 528, 590.
ischium, 47, 503, 535.
jugal, 477, 479, 5°2, 533-
lacrymal, 477, 501, 533,

753-

lesser multangular, 537.
lunate, 536.

49

Bone or bones —

magnum. See capitate,
malar. See jugal,
mandibular, 44.
maxillary, 44, 477, 478, 482,

488- 533- 753-
mentomeckelian, 44.
mesethmoid, 42, 501, 530,
590, 753-

mesocuneiform, 539.
mesopterygoid, 753.
metacarpal, 48, 502.
metapterygoid, 753.
metatarsal, 49, 503, 539.
multangular, greater, 537.
lesser, 537.

nasal, 42, 44, 477, 501, 530,
753-

navicular, 539.
navicular. See scaphoid,
occipital, 590.
of pentadactyle limb, 48.
opercular, 753.
opisthotic, 753.
orbitosphenoid, 501, 528,

590.

palatine, 43, 44, 488, 501,
532, 753.

parasphenoid, 41, 44, 501,
756.

parethmoid, 753.
parietal, 477, 478, 501, 528,

753-

patella, 539.
periotic, 529, 590.
phalanges, 49.
pisiform, 538.
postclavicular, 753.
postfrontal, 477, 478, 479.
postorbital, 477, 478, 479.
post-temporal, 753.
precoracoid, 45, 484, 486,

567-

prefrontal, 477, 478.
prehall ux, 49.

premaxillary, 43, 44, 477,
488, 501, 532, 733.
preopercular, 753.
presphenoid, 501, 528, 590.
prevomer, 42, 532.
prootic, 42, 44, 753.

770

INDEX

Bone or bones —
pterotic, 753.

pterygoid, 43, 44, 478, 488,
501, 532> 590, 753-
pubic, 47, 503, 535.
quadrate, 43, 478, 501.
quadrato- jugal, 44, 477, 502.
radial (radiale), 48, 536.
radio-ulna, 47.
radius, 48, 502, 536.
rostral, 501.
scaphoid, 536.
scapula, 45, 484, 502, 535.
semilunar. See lunate,
sphenethmoid, 41, 44.
sphenoid, 590-
sphenotic, 753.
splenial, 502, 756.
squamosal, 42, 43, 44, 477,
478, 479, 5°°> 501, 529>
590-

sternal. See Sternum,
subopercular, 753.
supraoccipital, 477, 500, 501,
528, 590, 753.
suprascapula, 45.
supratemporal, 477, 478,

481.

symplectic, 753.
talus. See astragalus,
tarsal, 49.
temporal, 590.
tibia, 49, 503, 539, 590.
tibiale, 49, 539. See also
astragalus,
tibio-fibula, 49.
transpalatine, 478.
trapezium. See greater
multangular.

trapezoid. See lesser mult-
angular,
triquetral, 536.
turbinal, 532.
tympanic, 530, 590.
ulna, 48, 502, 536.
ulnar (ulnare), 48, 536.
unciform. See hamate,
urohyal, 753.

vertebral. See Backbone,
vomer, 42, 44, 501, 532, 590,

753-

Bone or bones—

Wormian, 590.
zygomatic. See jugal.

See also Backbone, Girdle,
Ossicles, auditory and
brachial, Rays, Ribs,
Sternum.

Bones collectively. See
Skeleton.

cartilage and membrane, 40,
41, 44-

investing and replacing, 40.
Bony fishes, 463, 468.

Borneo, fauna of, 693.

Bot-flies, 343.

Botany, 30.

Botryoidal tissue, 284.

Bowels, 18.

Brachiolaria larva, 402,
Brachium, 34.

Brain, 18.

functions of, in frog, 96.

in Turbellaria, 240.
of Actinopterygii, 466.
of cockroach, 332.
of crayfish, 309.
of dogfish, 449, 450, 451.
of earthworm, 260.
of frog, 36, 84, 87, 89.
of liver fluke, 244.
of pigeon, 514, 514.
of Platyhelminthes, 239.
of rabbit, 558, 559, 560.
of reptiles, 484.
of Turbellaria, 240.

See also Ganglia, cerebral.
Branchiocardiac groove, 288.
Branchiostegite, 288, 307.
Bristles, 296, 520.

Bronchi, 75.
of frog, 75.

of pigeon, 506, 508, 509.
of rabbit, 547.

Bronchioles, 547.

Brown tubes oiAmphioxus, 415.
Buccal cavity. See Canal,
alimentary.

Buccinum, 682.

Bud. See Budding.

Budding, 175, 221, 222, 592.
of Amphioxus, 377.

INDEX

77i

Budding of Hydra, 221, 222,
223, 230, 231.
of Obelia, 230, 231.
of Protozoa, 175.
of Vorticella, 175.

Bugs, 338, 339.

Bunodont teeth, 574.

Bursa entiana, 436.

Bursa Fabricii, 506.

Butterflies, 350.

Butterfly, head of, 349.

white, 350, 351.

Byssus, 384.

Caeca, hepatic, of cockroach,
328.

lateral, of leech, 284.
pyloric, of cockroach, 329>

33°-

of cod, 751.
of starfish, 394.
rectal, of pigeon, 506.
of starfish, 394.

Ccscilia, 473, 685.

Caecum, hepatic, of Amphioxus,

4I3-

of crayfish, 302.
of man, 590.
of rabbit, 544, 546.

Calamus, 493-
Calf, fore-leg of, 572.
Calliphora, 348.

Camels, 575, 576- 577- 578-
Canal —

alimentary —

absent in tapeworm, 251.
of Amphioxus, 413.
of Ascaris, 356.
of cockroach, 328.
of cod, 751, 752.
of crayfish, 298.
of dogfish, 432.
of earthworm, 265.

of frog, 57-
of leech, 284, 284.
of liver fluke, 241, 243.
of Nereis, 280.
of pigeon, 505.
of rabbit, 539, 544, 546.
of starfish, 394-
of swan mussel, 376.

Canal — -

alimentary —

origin of, in chick, 651.
in tadpole, 628.
circular. See ring,
haemal, 426.

Laurer-Stieda, 241, 245.
neural, 609, 622.
neurenteric, 609, 610, 622.
pericardio-peritoneal, 432.
ring, 226.
stone, 394.

systems, of sponges, 203,
204, 205.

urinogenital, 551.
vertebral, 36, 38.
vertebrarterial, 524.
Canaliculi of bone, 122.

Canals —

aphodal, 206.
exhalant, 205.

Haversian, 122.
inhalant, 203.
radial, 226.

semicircular, 100, 101, 426.
Cannon bones, 576, 578.
Capillaries of frog’s web, 68.
Capitosaurus, 684.

Capitulum, 499, 526.

Capsules, Malpighian, 78.

nasal and auditory, 39, 41,
42, 426, 429, 629.
Carapace, of crayfish, 288.

of turtle, 485, 486.
Carbohydrates, 10, 27, 62, 144,
151, 217.

See also Glycogen, Para-
glycogen, Paramylum,
Starch, Sugar.

Carbon, circulation of, 714.
dioxide, 3, 4, 21, 73, 76, 217,
7I4-

oxidation of, 3.

Carbonic acid. See Carbon

dioxide.

Carchesium, 176, 177, 678.
Cardo, 324.

Carnivora, 583, 686.

Carotid gland. See Carotid

labyrinth,
labyrinth, 70.

772

INDEX

Carp, 467, 468.

Carpels, 24.

Carpopodite, 289.

Carpus, 34.
bones of, 48.

See also Skeleton of frog,
pigeon, rabbit.

Cartilage, 36, 119, 127.
arytaenoid, of frog, 75.
basihyal, of dogfish, 429.
calcified, 120.

ceratohyal, of dogfish, 429.
cricoid, of frog, 75.
epicoracoid, of frog, 45, 46.
fibrous, 1 19.
hyaline, 119.
hyoid, of frog, 44.
hyomandibular, of dogfish,
429.

Meckel’s, of frog, 44.
of dogfish, 429.
of rabbit, 533.
mesethmoid, of dogfish, 426.
palato-pterygo-quadrate, of
dogfish, 429.
of tadpole, 629.
postpublic, of frog, 47.
precoracoid, of frog, 45.
supradorsal, of dogfish, 426.
xiphoid, of frog, 46.
of rabbit, 527.
Cartilaginous fishes, 463.
Catalysts, 61.

Caterpillars, 351.

Cattle, 575, 576.

Cauda equina, 87.

Caul, 665.

Cavity, body. See Body cavity,
glenoid, 46.
subgerminal, 638.

Cavum aorticum, 66, 67.

pulmocutaneum, 66, 67.

Ceil, diagram of, 104.
division, 128.
theory, 149, 150.

Cells, 23, 103, 104, 125, 149.
blood, 64, 122, 123, 130.
bone, 122.

central, of nephrostome, 267.
chloragogenous, 266.
ciliated, 109, 109.

Cells, cystogenous, 248.
differentiation of, 125.
effector, 93.
epithelial, 109.
fat, 120, 129.
flame, 243, 244.
germ, 113, 214.
gland, hi, 111.
glandular, of Hydra, 216.
goblet, 107, hi.
interstitial, 210.
lenticular, 31 1.
lower layer, 639.
marginal, of nephrostome,
267.

multinucleate, 103.
muscle, 1 1 7.
musculo-epithelial, 209.
nerve, 115, 116, 116, 214.
nutritive, of Hydra, 217.
of Rauber, 660.
phagocytic, 355.
pigment, 35, 63.
plant, 25.
pole, 281, 667.
receptor, 93.

sense, of earthworm, 265.
sensory, 109, 110, 214.
visual, 31 1. See also Retina,
yellow, 266, 267.

Cellular animals, 177.

Cellulose, 28.

digestion of, in herbivores,
546.

Centipedes, 316, 681.
Centrolecithal cleavage, 666,
668.

Centrosome, 108, 128, 137.
in spermatozoon, not in
ovum, 1 13, 1 14.
Cephalisation, 277, 280, 310.
Cephalochorda, 683.
Cephalopoda, 385, 682.
Cephalothorax, of crayfish,
287.

of spiders, 318.

Cercarise, 248, 249.

Cerci anales, 325.

Cere, 491.

Cerebellum, 87.
of dogfish, 452.

INDEX

773

Cerebellum, of frog, 87.
of pigeon, 514.
of rabbit, 563.

Cerebral convolutions, 5^°>
588.

fissures, 560.

ganglia. See Ganglia, cere-
bral.

hemispheres, 87.

joined in dogfish, 449- -
of frog, 87.
of pigeon, 514, 514.
of rabbit, 560.
Cerebro-spinal axis, 84.
nerves, 84.
system, 84.

Cerebrum, of dogfish,

449- •• ~

of Actinopterygn, 466.

See also Cerebral hemi-
spheres.

Cervical groove of crayfish,
288.

Cestoda, 249, 679.

Cetacea, 568, 686.

Chaetse of earthworm, 257, 258,
258, 262, 520.
of Nereis, 280.

Changes during development,
i3> J4-

Chelicerae of Acarma, 318.
of Ixodes, 320.
of spiders, 318.

Chelipeds, 295.

Chelonia, 485, 685.

Chemical activity in plants,
25-29, 217.

work in the body, 6, 7,

x7-

Chemosynthesis, 714.

Chiasma, optic, 90.
of dogfish, 452.
of frog, 90-

Chick, circulation of, 650.
development of, 638.
stages in, 640.
heart in, 656.
embryo (36 hrs.), 498.

diagram of layers and
folds of, 495.
Chiroptera, 583. 68°-

Chlamydomonas , 26,28, 152,153,
154, 156, 677, 713, 739-
syngamy in, 153.
Chlamydospores, 159.
Chlorophyll, 24, 25, 217.

role of, in plants, 26.
Chloroplasts, 24, 154.
Choanichthyes, 463, 472, 684.
Choanocytes, 202.
Choanoflagellata, 156, 200, 677.
Chondrin, 120.

Chordae tendineae, 66, 512, 553.
Chordata, 418, 622, 668, 682.
Chords, vocal, 75.

Chorion, 649.

Choroid coat, 99.

plexus, 87, 88.

Chromatids, 129.

Chromatin, 108, 129, 197.
Chromidial chromatin, 197.
Chromomere, 129.
Chromosomes, 108, 129, 669,
699, 700.
daughter, 129.
homologous, 132.

Cilia, 109, 161, 163.

movements of, 153.

Ciliata, 161, 177, 178, 678.
conjugation in, 170.
in frog and man, 195, I97>
198.

reproduction of, 594.

Cimex lectularius, 338> 339.
Circulation of the blood, 69, 70.
in Amphioxus, 415-
in crayfish, 305.
in dogfish, 442-449, 449.
in earthworm, 269, 270.
in frog, 69, 70, 73, 74, 76.
in rabbit, 553, 554, 557.
in swan mussel, 382.
regulation of, 70, 92, 564.
of carbon, 714.
of nitrogen, 714.
organs of, 18. See also Vas-
cular system.

Cirri of Amphioxus, 406.

of Nereis, 280.

Cirrus of liver fluke, 245.
sac, 245.

Clasper, 420, 432.

77 4

INDEX

Class, 677.

Classes, definitions of, 677.
Classification, 675.

suggests evolution, 689, 690.
table of, 677.

Clavicles, 45.
of frog, 45.
of pigeon, 502.
of Primates, 584.
of rabbit, 535.

Claws, 18, 518.

Cleavage of ovum, 137.

complete or holoblastic,

637- 657. 666-
centrolecithal, 668.
determinate, 670.
incomplete or meroblastic,
637-

indeterminate, 670.
kinds of, 637.

of Amphioxus, 606, 607, 637.

of bird, 638.

of dogfish, 637.

of frog, 137, 616, 637.

of Hydra, 220.

of invertebrates, 666, 667.

of mammals, 657.

Clitellum, 257, 273, 274.
Clitoris of rabbit, 518, 551.
Cloaca, 58.

of dogfish, 420, 436.
of frog, 58.
of Monotremata, 567.
of pigeon, 506.
wanting in adult rabbit, 518,
548-

Cloacal opening. See Vent.

space of mussel, 376.
Clotting of blood, 127.

Clypeus, 322.

Cnemial crest, 539.
Cnidoblasts, 210, 212, 214.
Cnidocil, 210, 212.

Coagulate, 107.

Coccyx, 583.

Cochlea, 101, 565.

Cockle worm, 360.

Cockroach, 322, 323, 324, 327,
744-

abdomen of, 325.
alimentary systems of, 328.

Cockroach, anatomy of, 322,

333.

blood vessels of, 331.
digestion and excretion of,
329-

head and thorax of, dissec-
tion of, 331.
head of, 322, 325.
locomotion of, 327, 328.
mouth appendages of, 323,
324, 326.

nervous system of, 332.
reproductive organs of, 334,

336, 337.

respiratory organs of, 330.
sense organs of, 333.
thorax of, 324, 325.
tracheal tissue of, 335.

Cod, 468, 750.

Coelenterata, 233, 238, 403,

678.

Coelom, 275. See also Per-
visceral cavity, coe-
lomic.

of Amphioxus, 407, 413,

416.

of arthropods, 680.
of bird, 643, 649.
of crayfish, 298, 308.
of dogfish, 421, 432.
of earthworm, 260, 275.
of frog, 35, 275, 621, 628.
of leech, 286.
of rabbit, 521, 546.
of starfish, 392.
of swan mussel, 376.
origin of, in ontogeny, 613,
614.

Coelomata, 680.

Ccelomic epithelium, of earth-
worm, 260, 266.
of frog, 1 1 3, 628.
of swan mussel, 381.

See also Peritoneum.
Coelomic fluid, 36.
Coelomoducts, 275, 308, 414.
Coenocyte, 103, 109, 116, 147
Coenosarc, 224.

Ccenurus cerebralis, 254.

Coffin bone, 574, 578.

Coition, 16, 551, 600.

INDEX

775

Cold-blooded animals, 74, 460.

vertebrates, 460.

Coleoptera, 342, 681.

Colloids, 105.

Colon of rabbit, 546.

Colonies, 177, 223.

Columba, 490, 695.

Colour changes in frog, 35, 64.
Columella auris, 42, 101.

of mollusca, 385.

Columnae carneae, 68.
Combustion in the body, 4.
of carbohydrates and fats,
10.

Commensalism, 716.
Commissures, cerebral and
cerebro-pedal, of mus-
sel, 383- , ,

circumoesophageal, ox cock-
roach, 332.
of crayfish, 309.
circumpharyngeal, of earth-
worm, 260.
of leech, 286.

nervous, of crayfish, 309, 310.
of leech, 286.
visceral, of mussel, 384.
Concretion, calcareous, round
spinal nerve, 86.
Conduction, 17.

in nerve, 7, 22, 93, 94-
in nerve net, 215.
Conductivity, 7, 126.
in Amceba, 144.
in leucocytes, 126.
in nervous tissue, 20, 94,
126.

in plants, 24.

Cone, oral, 209, 216.

Cones of retina, 110, m.
Conjugants, 168, 176, 177.
Conjugation, 16, 594. See also
Syngamy.
in Ciliata, 170.
of Paramecium, 168—171,
598.

of Vorticella, 176.
Conjunctiva, 98.

Connective tissue, 120.
Consciousness, 97.

Continuity of life, 726.

Contractile fibre of Vorticella,

174.

processes of cells of Hydra,
209.

Contraction, 6, 17, 20, 49, 117,
141.

Conus arteriosus, 66, 442, 632.
Convolutions, cerebral, 560,
588.

Coprodasum, 506.

Copromonas, 15, i55> I5^> ^77*
food of, 155.
syngamy in, 155.

Copulation, 16

Coracoid, 45- See also Girdle,
pectoral.

“ Coral insect,” 234.

Corals, 234, 679.

Cord. See Spinal cord, Nerve
cord, Spermatic cord,
Umbilical cord.

Cords, vocal, 75.

Cornea, 98.

of crayfish, 31 1.
of frog, 98.

Coronary cushion, 574.

Corpora quadrigemina, 563.

striata, 89, 514.

Corpus albicans, or mam-
millare, 562.
callosum, 560.

Corpuscles, blood, 122, 130,
See also Blood,
bone. See Bone cells,
connective tissue, 120.
Cortex, cerebral, 89.
in frog, 89.
in rabbit, 560.
of Paramecium, 162.
of adrenal bodies, 63.
of green glands, 308.

Costa of wing, 327.

Costal plates of turtles, 486.
Coverts, 493.

Coxopodite, 289, 293.

Crab, hermit, 716, 717-
Crab-louse, 339. 34°. 341.
Crabs, 316.

Crane-flies, 343-
Cranial flexure, 626, 657.
nerves. See Nerves, cranial.

776

INDEX

Cranium, 39, 426. See also Skull.
Crawling, 56, 56, 476. See also
Locomotion.

Crayfish, 287, 288, 290, 291,
301, 317, 743.
abdomen of, 288, 297,
alimentary system of, 298.
blood vessels of, 302.
cuticle of, 296.
development of, 315, 666,
667.

epidermis of, 296.
excretory organs of, 308.
eye of, 31 1, 312.
female, 290.

habits and external features
of, 287.
limbs of, 289.
locomotion of, 297, 298.
mouth appendages of, 293.
nervous system of, 309.
podobranch of, 306, 307.
reproduction of, 314.
reproductive organs of, 314,
3i5-

respiratory organs of, 306.
segments of, table of, 296.
sense organs of, 31 1.
skeleton and muscles of,
297-

thorax of, 288.
walking leg of, 294.

Crest, cnemial, 539.

Cretins, 673, 674.

Crinoidea, 401, 403.

Crithidial phase, 187.
Crocodiles, 481, 487.

skull of, 483.

Crocodilia, 487, 685.

Crop of cockroach, 328, 329.
of earthworm, 265.
of leech, 284.
of pigeon, 505.
Cross-fertilisation, 158, 602.
Crossing-over, 133.
Crossopterygii, 472, 473, 474,
684.

Crown of tooth, 57.

Crura cerebri, 87, 563.

Crus, 34.

Crustacea, 316, 680.

Crystalline style, 377.
cone, 31 1.

Crystalloids, 105.

Culex, 196, 343.

Cuticle, 151.

of Annelida, 282.
of Arthropoda, 316.
of Ascaris, 354.
of cockroach, 322.
of crayfish, 296.
of earthworm, 260.
of Nematoda, 368.
of Polytoma, 151.

Cuttlefish, 385, 388.

Cycle, life, 14, 14.

of various animals. See
Life-history.

Cycloid scales, 468.

Cyclops, 316, 361, 362, 366, 680.

Cyclostomata, 460, 683.

Cyst of Amoeba, 145, 147.
of Cercaria, 248.
of Entamosbz , 180, 183.
of Monocystis, 158.
of Opalina, 197, 198.
of Polytoma, 152.
of Trichinella, 363.

Cysticercus, 252.

Cystogenous cells, 248.

Cytophore, 273.

Cytoplasm, 103, 669.

Dactylopodite, 289.

Darwinism, 687, 705.

Deamination, 10.

Death, 138, 150, 238.
in the Ccelenterata, 238.

Deep sea fauna, 720.

Dehydration, 730.

Delamination, 643, 666 fn.

Deltoid ridge, 47.

Demodex, 681.

Demodex folliculorum, 319, 319.

Dendrites, 115.

Dens of epistropheus, 524.

Dental formula, 541.

Dentine, of teeth, 57, 422.

Depression, 171.
in Amoeba, 145.
in Hydra, 219, 238.
in Paramecium, 171, 598.

INDEX

777

Dermis, 34, 35, 120.

Dermotrichia, 431, 432.

Determination of sex, 601.

Development, 14.
of birds, 638, 658.
of crayfish, 315, 666, 667,
668.

of dogfish, 442, 623.
of earthworm, 667.
of frog, 31, 616.
of Hydra, 220, 222, 666.
of invertebrates, 666.
of lancelet, 605.
of liver fluke, 245.
of mammals, 657.
of Obelia, 228, 229, 666.
of swan mussel, 384.

See also Embryology, Life-
history.

Diaphragm, 521.

Diastema, 541.

Differences between animais,
20, 687.

animals and plants, 24, 29.
parents and offspring, 13.

Differentiation, 19, 20.
of cells, 125.

Digestion, 9, 10, 11.
in Amoeba, 143.
in Amphioxus, 414.
in crayfish, 302.
in earthworm, 267.
in frog, 61.
in Hydra, 219.
in mussel, 376, 377, 380.
in Paramecium, 165.
in pigeon, 505.
in rabbit, 546.
in snail, 388.
in starfish, 394.
in Vorticella, 175.

Digestive organs, 18. See also
Canal, alimentary.

Digitigrade, 574.

Digits, 34.

Dimorphism of gametes, 15,
600, 601.
of sexes, 601.

Diploblastica, 275, 678.

Diplotene stage, 134.

Dipnoi, 472, 684.

Diptera, 343, 681.

Disc, of starfish, 389.

of Vorticella, 173.

Discs, imaginal, 338.
Disintegration of substances
in body, 3, 17.
Dissecting dish, 729.

instruments, 729.

Dissection, 730, 733.

Distomum, 679.

hepaticum, 241, 241, 242,
243.

Distribution of animals, 692,
719.

Division, amitotic, 128.
meiotic, 131.
mitotic, 128, 131.
of Amoeba, 146.
of cells, 128.
of nuclei, 128.
of Paramecium, 167.
post-meiotic, 134.
reducing, 131.

See also Fission.

Dog, dental formula of, 583.

skull of, 528, 580, 581.
Dogfish, 419, 420, 749-

alimentary system of, 432.
backbone of, 424, 425.
blood vessels of, 441, 442>
443, 445.

brain of, 450, 451.

excretory organs of, 437.
external features of, 419.
generative organs of, 438,
439-

heart of, 442.
limbs of, 420, 431.
muscles of, 422, 423.
nervous system of, 428, 449*
452.

sense organs of, 428, 456.
skeleton of, 424, 427.
skin of, 422.
skull of, 425, 426.
veins of, 446, 447, 448.
Dogs, 583.

Dolphins, 568.

Dominance, 138, 598, 697.
Dorsal pores, 259.
surface, 33.

778

INDEX

Dracunculus medinensis, 361,

362.

Dragon-flies, 338.

Drawing, 730.

Drosophila, 700, 701, 702.

Drugs, effect of, on Para-
mecium, 165, 166.
on protoplasm, 106.

See also references to various
parasites.

Duckmole, 566, 567.

Duct, 6, 59, 1 1 2.

archinephric, 635, 652.
bile, 59.

of dogfish, 436.
of frog, 59.
of man, 590.
of pigeon, 506.
of rabbit, 544.

mesonephric or Wolffian,
484, 652, 653.
of dogfish, 437, 438.
of frog, 78, 79.
of frog embryo, 636,
of rabbit, 547, 548.
metanephric. See Ureter.
Mullerian, 636, 653.
nasal, 564.

pancreatic, of dogfish, 436.
of man, 586, 590.
of pigeon, 506.
of rabbit, 544.
segmental, 635, 636, 652.
thoracic, 557.

Ductless- glands. See Glands,
ductless.

Ductus arteriosus, 475, 484,
556, 635.

Botalli, 635.

caroticus, 475, 484, 633,

635-

Cuvieri, 416, 446, 448, 633.
deferens. See Vas deferens,
ejaculatorius, of cockroach,
334-

endolymphaticus, 101.
of dogfish, 426.
of frog, 1 01.
of rabbit, 565.
venosus, 654.

Dung, 11.

Duodenum, 58.
of dogfish, 436.
of frog, 58.
of man, 586.
of pigeon, 506.
of rabbit, 543, 544.

Dwarfs, 673.

Ear, 18.

of dogfish, 458.

of frog, 33, 100, 100, 101.

of pigeon, 514.

of rabbit, 529, 530, 562, 564.

Earthworm, 255, 255, 257, 741.
alimentary canal of, 265-
267.

circulation of, 269, 270, 271.
development of, 667.
dissection of, 263.
excretion in, 267.
external features of, 256.

openings of, 258.
food of, 256.
habits of, 255.
histology of, 260, 266.
mechanism of locomotion
of, 262, 264.

nephridium of, 267, 268, 269.
nervous system of, 260, 262,
264.

pairing of, 257, 273.
regeneration in, 275.
reproduction of, 270.
reproductive organs of, 274.
section of, 259.
sense organs of, 265.

Echinococcus, 254.

Echinoidea, 400, 403.

Echinodermata, 403, 682.

Ectoderm, 275.

of Hydra, 209, 275.
of Tripoblastica, 275. Seealso
Epiblast, Epidermis.

Ectoplasm, 140.

of Amoeba, 140, 141.
of Entamoeba, 179, 180.
of Monocystis, 157.
of Paramecium, 162.
of V orticella, 174.

Edentata, 571, 686. i

Eel, 469.

INDEX

779

Effector cells, 93-
Efferent or motor fibres, 93.
nerves, 92, 96.
root, 86.

Egg. See Ovum.

sac of earthworm. See Re-
ceptaculum ovorum.
of frog, 82.

Elasmobranchn, 463, 471, 663.
Elastic fibres, 120.

Elastin, 431. .

Electricity, production of, by
living beings, 7, 93-
Elephants, 574, 5Sl-
Emboly, 666.

Embryo, 32.

folding off of the, 644.
Embryology, 605. See also De-
velopment, Life-history,
suggests evolution, 692. >
Embryonal. See Embryonic.
Embryonic area, 660.

circulation of chick, 654-
of rabbit, 663, 664.
membranes, 644. See also
Allantois, Amnion,
nutrition, 665.
plate, 660.
shield, 660.

Encystment. See Cyst.
Enamel, 57, 422, 578.
Enchylema, 104.

End sac of crayfish, 308.
Endocrine organs. See Duct-
less glands.

Endoderm, 275.

of Hydra, 209, 216, 275.
of Tripoblastica, 275. See
also Hypoblast.
Endolymph, 100, 102.
Endomixis, 172.
Endophragmal skeleton, 297.
Endoplasm, 140.
of Amoeba, 140.
of Monocystis, 157.
of Paramecium, 162, 163.
of V orticella, 174.
Endopodite, 289.
Endoskeleton, 424.

Endostyle, 412, 463.

Energid, 109, 149-

Energy, of life, 2, 17, 714, 7x5>
721, 723-
forms of, 5, 6, 7.
Energy-producing factors m
food, 10.

storage of, by plants, 29.
Entamoeba, 179, 180, 182, 183,
678.
coli, 179-

histolytica, 180, 181, 182, 183.
life cycle of, 181.

Enteron, 610.
of Hydra, 209.
of Obelia, 228.

See also Archenteron.
Enterostome, 406.

Eohippus, 695.

Enzymes, 10, 61, 713.

Epeira, 318, 681.

Ephyra, 236.

Epiblast, 607, 639.

Epiboly, 617, 666.
Epibranchial space, 375.
Epicranial plates of cockroach,
322-

Epidermis, 34, 275.
of crayfish, 296.
of earthworm, 260.
of frog, 113, 275.
of leech, 282.
of liver fluke, 241.
of pigeon, 492.
of rabbit, 520.

Epididymis, 484, 550, 653.

of leech, 286.

Epiglottis, 543.

Epimere, 613, 614, 621, 629.
Epimeron, 288.

Epipharyngeal groove, 412.
Epipharynx, 323.

Epiphragm of snail, 388.
Epiphysis, 524.

Epipodite, 289.

Episternum, 46.
Epistropheus, 524.
Epithelium, 109.
ciliated, 109.
columnar, 109.
cubical, 1 12.
germinal, 113.
glandular, in.

780

INDEX

Epithelium of alimentary
canal. See Endoderm.
of skin. See Ectoderm,
pavement, 112.
sensory, 109.
simple, 109.
stratified, 109, 113.

Epoophoron, 653.

Ethmoidal region, 40.

Euglena, 154, 154, 155, 156,

677< 739-
viridis, 154, 155.

chloroplasts of, 154, 155.
nutrition of, 155.
reproduction of, 155.

Euryhaline animals, 396.

Euspongia, 205, 206, 206, 678.
officinalis, 206.

Eustachian canal, 535.

tube, 58, 101, 505, 540, 628.
valve, 654.

Eutheria, 568, 686.

Evolution, 687.
evidence of, 689.
mode of, 695.
the organism in, 710.
theories of, 703.

Examination of specimens,
732, 733-

Exconjugants, 169.

Excretion, 7, 17, 712. See also
Excretory organs.

Excretory organs, 19.
of Amoeba, 145.
of Amphioxus, 414.
of Ascaris, 354, 337.
of crayfish, 308.
of dogfish, 437.
of frog, 73, 73.
of Hydra, 219.
of leech, 284.
of liver fluke, 243.
of mussel, 380.
of Paramecium, 164.
of tadpole, 635.
of tapeworm, 251.
of V orticella, 175.

See also Kidneys.

Exopodite, 289.

Exoskeleton, 424.

Extinction of species, 689.

Exumbrella, 224.

Eye muscles of dogfish, 456,
457-

of frog, 99.

Eyelashes of rabbit, 518.

Eyelid, third, 33, 492, 518.

Eyelids, 33, 458, 518.
of dogfish, 458.
of frog, 33.
of pigeon, 492.
of rabbit, 518.

Eyes, 18.

of cockroach, 333.
of crayfish, 289, 31 1, 312.
of frog, 33, 97, 98, 99-
of Miracidium, 245.
of Nereis, 280.
of plaice, 470.

Eyespot. See Stigma.

Facet, acetabular, 432.
for rib, 526.

for transverse process, 526.
glenoid, 431.

Faeces, 11.

Fallopian tubes, 551.

Family, 677.

Fascia, dorsal, of frog, 51.

Fat bodies, of frog, 64.

Fats, 10.

Fatty body of cockroach, 332.

Faunas, 692, 719.
geographical, 719.
oecological, 719.
pelagic, 718, 720.
snow, Frontispiece.

Feathers, 492, 493, 494, 495.

Feet of elephants, 581.
of mammals, 572, 574.
of ungulates, 574-576, 578.

Female, 15, 600.

gamete. See Gametes.

Femur, 34, 49. See also Bone,
femur.

Fenestra ovalis, 42, 101, 530.
rotunda, 530.

Fenestrae of cockroach, 333.

Fermentation, 713.

Ferments, 61.

Fertilisation, 15, 136.
cross-, 158, 602.

INDEX

781

Fertilisation, self-, 158.

See also Reproduction.

Fibrin, 127.

Fibrinogen, 127.

Filaria, 362, 363, 680.
bancrofti, 362, 363.
diurna, 363.
loa, 363-
nocturna, 363.
pevstans, 363.

Filoplumes, 493-
Filter - chamber of crayfish,
299.

Filum terminale, 84.

Fins, 18.

of Amphioxus, 404.
of cod, 751.

of dogfish, 420, 423, 424.
of Teleostomi, 468.
unpaired, 474.

Fishes, 460, 463.

Fission, 13, 14, *7* 23I> 232>
233. 592.
binary —

in Amoeba, 145, 146.
in Balantidium, 196.
in Entamoeba, 179, 181.
in Euglena, 155.
in Hydra, 222.
in Paramecium, 166, 171.
in Polytoma, 152.
in Trypanosoma, 186.
in Vorticella, 175.
budding, 175.
in Protozoa, 175 fn.
multiple, 146, 147, 175-
repeated, 152, 175.

Fixing, 731.

Flagella, 28, 15 1.

movements of, 153, 177.
Flagellata, 28, 151, 156, I77-
See also Mastigophora.
Flagellated chambers of
sponges, 204.
Flagellispores, 159-
Flagellulse, 159.

Flame cells, 243, 244,

Flat fishes, 469.

Flatworms. See Platyhel-
minthes.

Flea, 348, 349, 365-

Flight of cockroach, 327.
of pigeon, 494-496.

Flocculus, 563.

Fluke, liver, 241, 740.

alimentary system of, 241.
excretory system of, 243.
generative organs of, 244,
246.

life-history of, 245, 247,
249.

nervous system of, 244.
structure of, 243.

Foetus, 661.

Folding off of the embryo, 644.

Folds, head and tail, 646, 661.

Follicle of ovum, 80, 114, 550.

Fontanelle, anterior, of dogfish,
426, 429.
coracoid, 45.

Fontanelles of frog’s skull, 41.

Food, 8, 217, 218.
of Ammoccetes, 463.
of Amoeba, 143, 144-
of Ascaris, 357.
of cockroach, 322, 329.
of crayfish, 287.
of dogfish, 419.
of earthworm, 256.
of Entamoeba, 179, 181.
of frog, 32.
of Hydra, 217, 218.
of lamprey, 461.
of leech, 282.
of liver fluke, 241.
of Monocystis, 157, 160.
of Paramecium, 161, 164.
of parasites, 366.
of pigeon, 490.
of plants, 24, 27, 217.
of Polytoma, 15 1.
of protoplasm, 27, 217.
of rabbit, 517.
of sponges, 203.
of starfish, 394.
of swan mussel, 374.
of tadpole, 32.
of Vorticella, 175-
Foot of frog, 34.
of Hydra, 208.
of pigeon, 491, 503, 504.
of rabbit, 518, 538, 539-

7S2

INDEX

Foot of swan mussel, 369, 372.

skeleton of, 49.

Foot. See also Feet of mammals.
Foramen, intervertebral, 39.
magnum, 40.
of dogfish, 429.
of frog, 40.
of pigeon, 500.
of rabbit, 528.
obturator, 48, 503.
of Monro, 89.
ovale, 654.
triosseum, 502, 503.
Foramina, cranial, of frog, 41.
of skull, in dogfish, 429.
in rabbit, 533, 534.
Fore-gut, 300, 328, 628. See
also Stomodaeum.
Fore-leg of calf, 572.
of pig, 572.
of pony, 579.

Fore-limb of Balcenoptera , 571.
Formaldehyde, 27.

Formed material, 22.

Fossa, mandibular, of frog, 44.

pituitary, 629.

Fossae of reptiles, 478, 481.
Frenulum, 351.

Frog, 31, 32.

alimentary system of, 57.
arterial system of, 70, 71.
brain of, 87, 89.
cleavage of ovum of, 616,617.
cranial nerves of, 41, 90.
death of, 138.
dissection of male, 60.
embryology of, 616.
excretory organs of, 78.
external features and body-
wall of, 33.

gastrulation of ovum of, 617,
618.

hand of, 34.
heart of, 65, 65, 66, 67.
histology of, 103.
life-history of, 31.
locomotion of, 31, 32, 56.
muscular system of, 49, 53.
nerve fibres entering and
leaving cord of, 117.
nervous system of, 84, 85.

Frog, reproductive organs of, 80.
respiration of, 75, 76.
sense organs of, 98.
skeleton of, 36, 37.
spinal cord and nerves of, 88.
teeth of, 57, 57, 58.
urinary and generative sys-
tem of, 77, 78, 80, 81, 82.
vascular system of, 65.
venous system of, 72.
ventral dissection of, 60,
vertebrae of, 36, 38.
viscera of, 57.

Frons, 322.

Frontal lobe, 560.

Function in relation to struc-
ture, 18, 30.

the stimulus to developing
rudiments, 672.

Fur, 517.

Gadus, 468, 684, 750.

Galea, 324.

Gall-bladder, 59.

Gametes, 15, 596.

dimorphism of, 15, 601.
male and female, 15, 601.
maturation of, 130.
of flagellata, 1 52-154.
of frog, 115, 130.
of malaria parasite, 190.
of man, 13, 16.
of Monocystis, 158.
of Paramecium, 168.
protozoan, ripening of, 199.
See also Ovum, Spermato-
zoon.

Gametocyte, 131, 190.

Gametogenesis, 130, 135.

Garnont, 190.

Ganglion, ganglia, 84, 116.
cerebral, of mussel, 383.
cerebral. See also Supra-
pharyngeal and supra-
oesophageal.

cervical sympathetic, 564.
coeliac, 563.
dorsal root, 86.

Gasserian, 90, 92.
geniculate, 90 fn., 92.
of cockroach, 332.

INDEX

7*3

Ganglion, ganglia of crayfish, 309
parietosplanchnic, 384.
pedal, 383.
petrosal, 91.

suboesophageal, of cock-
roach, 332.
of crayfish, 309.
subpharyngeal, of earth-
worm, 260.

supraoesophageal, of cock-
roach, 332.
of crayfish, 309.
suprapharyngeal, of earth-
worm, 260.

sympathetic, of frog, 92.

of rabbit, 563, 564.
thoracic, of crayfish, 309.
vagus, of frog, 91, 92.

of rabbit, 564.
visceral, of mussel, 384.
Ganoid scales, 468.

Ganoids, 466, 684.

Gape, 57.

Gastric glands, 59, hi.
juice, 7, 61.
mill of crayfish, 300.
ossicles, 300.

Gastroliths, 302.

Gastropoda, 385, 682.

Gastrula, 607.

of Amphioxus, 607.
of bird, 643.

of certain invertebrates, 666,
667.

of crayfish, 667.
of frog, 617.
of mammals, 657.
Gastrulation, 617, 657.
modes of, 666 fn.

of Amphioxus, 606, 607.
of bird, 643.

of certain invertebrates,
666, 667.
of frog, 617, 618.

Gavials, 487.

Gelatin, 120, 122.

Gels, 105.

Genae, 322.

Gene complex, 702.
Genealogical tree, 690.

Genera, 677.

Generative (or reproductive)
organs, 19.
of Amphioxus , 418.
of Ascaris, 357.
of cockroach, 334, 336, 337.
of crayfish, 314, 315.
of dogfish, 438, 439.
of earthworm, 270, 273, 274.
of frog, 80, 635, 636.
of Hydra, 219,' 22i.
of leech, 286.
of liver fluke, 244, 246.
of lizard, 755, 756.
of mammals, female, 568,
568.

of medusae, 228, 233, 236.
of Nereis, 280.
of pigeon, 508, 509, 510.
of rabbit, 548, 550.
of sea-anemone, 233.
of snails, 387.
of swan mussel, 384.
of tapeworm, 252.
of Teleostei, 469.

Genes, 698, 701, 709.

Genital organs. See Genera-
tive organs.

pouch of cockroach, 326.
Geological record, 694.

Germ, 15, 593. 698, 726. See
also Gametes,
layers, 610, 625, 639.
Germinal disc, 637.
vesicle, 605.
wall, 639.

Giant fibres, 261, 264.

Giants, 673.

Gibbon, the, 585.

Gill clefts, 419.

of Amphioxus, 406, 410, 418.
of dogfish, 419, 421, 435.
of tadpoles, 623, 628.
of Teleostomi, 466.

See also Visceral clefts.

Gill cover of crayfish, 288.

of fishes. See Operculum.
Gill plate, 622, 623.

slits. See Gill clefts.

Gills, 19.

dermal, of starfish, 389.
of crayfish, 288, 306, 307.

784

INDEX

Gills of dogfish, 435, 442.
of mussel, 374, 375, 376.
of tadpole, 442, 623.
of Teleostomi, 466.

Girdle, pectoral, 36, 45.
of cod, 753, 754.
of dogfish, 431.
of duckmole, 566.
of frog, 36, 45, 46.
of lizard, 759.
of Monotremata, 567.
of pigeon, 502.
of rabbit, 529, 535.
of turtle, 486.
pelvic, 36, 46.
of cod, 753.
of dogfish, 432.
of frog, 36, 46.
of pigeon, 503.
of rabbit, 535, 536.

Girdle, limb, 36.

of pentadactyle animals, 48,
49.

Gizzard of cockroach, 328.
of earthworm, 265, 267.
of pigeon, 505.

Gland (s), 6, hi, 111, 112.
adrenal, 548.
carotid, 70.
clitellar, 286.
club-shaped, 613.
colleterial, 334.
conglobate, 334.

Cowper’s, 551.
ductless, 21, 62.
of dogfish, 458.
of frog, 62.
of rabbit, 546.
grease, 520.
green, 293, 308, 668.
Harderian, 98, 564.
infraorbital, 543.
ink, 388.
lacrymal, 564.
milk, 518, 566.
multicellular, in.
mushroom-shaped, 334.
oesophageal, 265.
oil, 492.
parotid, 541.
perineal, 518, 550.

Gland (s), prostate, 551.
racemose, 112.
rectal, 436.

salivary, of cockroach, 328.
of rabbit, 541.
of snail, 388.
sebaceous, 520.
shell, of dogfish, 440.

of liver fluke, 245.
silk, 351.
stink, 326.
sublingual, 543.
submaxillary, 541.
suprarenal, 63, 64, 548.
sweat, 520.
thyroid, 63.
tubular, 112.
unicellular, in.
yolk, 245.

See also Liver, Pancreas, etc.
Glenoid cavity, 46, 535.
facet, 431.

Glochidia, 382, 383, 384.
Glomeruli, of kidney, 78, 633,
635-

Glomus, 635.

Glossa, 324.

Glossina morsitans, 186.

palpalis, 186, 187.

Glottis, of frog, 58.
of rabbit, 543.

Glucose, disintegration of, 3, 10.
Glycogen, 4, 62, 151, 163, 367.
Gnathostomata, 429, 531.
Gnats, 195, 196, 343. “

Golgi bodies, 108.

Gonads, 80.

development of, in chick,
652, 653.

See also Ovaries, Testes.
Gonangium, 229.
Gonapophyses, 326.

Gonocoele, 418.

Gonotome, 615.

Gonotheca, 229.

Graafian follicles, 551.
Grasshoppers, 338.

Green bodies of Hydra viridis,
217.

Green colour in plants, 24, 29,
217.

INDEX

785

Green glands of crayfish, 308.
Grey matter, 84, 89, 116.
Groove, cervical, 288.
epibranchial, 412, 413.
oronasal, 419.
primitive, 623, 642.

Growth, 2, 12, 14.

changes in structure during,
14.

Gvyphcea, 710.

Gubernaculum, 550.

Guinea worm, 361,

Gullet of Balantidium, 195,
197.

of frog, 58.
of Nyctotherus, 197.
of Paramecium, 161.
of Vorticella, 173, 175.

See also (Esophagus.

Gut. See Alimentary canal.
Gymnophiona, 474.
Gymnospores, 159.

Haddock, 468, 750.
Hcemambeba, 188.

Haemoccele, 275, 306.

See also Perivisceral cavity,
haemocoelic ; Vascular
system.

Haemocyanin, 306, 388.
Haemoglobin, 123, 269.

Hairs of mammals, 520, 566.

of rabbit, 520.

Halteres, 336, 343.

Haltica nemorum, 342.

Hand, 34. See also Limb,
.fore ; Feet of mammals,
of frog, 34.

Hardening, 732.

Harderian glands, of frog,
98.

of rabbit, 564.

Hare, the, 518, 520, 677.

“ Hare lip ” of rabbit, 518.
Hatching of chick, 657.

of lancelet, 61 1 .

Hatschek’s nephridium, 414.

pit, 406, 613.

Haversian canals, 122.

Head, 277, 280.
of frog, 33, 59.

50

Heart, 18.

of cockroach, 331.
of cod, 469.
of crayfish, 302.
of dogfish, 442.
of frog, 65.
of pigeon, 512.
of rabbit, 552, 553.
of reptiles, 481.
of skate, 463.
of swan mussel, 382.

Heart-and-dart moth, 350, 351.

Heart-beat, 68, 554.

regulation of, 70, 92, 564.

Hearts of earthworm, 270.
lymph-, of frog, 75.

Heat production, 7, 74.

Helix, 682, 746, 747.
pomatia, 386, 387, 746,

Hemiptera, 338, 681.

Hepatic caeca of cockroach, 328.

Hepato-pancreas, 302.

Heredity, 14, 137, 138, 598,
599- 7°8-

Hermaphrodite duct, 387.

Hermaphrodites, 15, 168, 239,
244, 270, 286, 365.

Hermaphroditism, 15, 601, 602.

Hermit crabs, 716, 717.

Herring, 468.

Heterocoelous, 499.

Heterogamy, 249.

Heterometabola, 336, 681.

Heterozygote, 699.

Hexapoda. See Insects.

Higher organisms, 20,

Hind-gut, 300, 320, 628. See
also Proctodaeum.

Hippocampus, 562.

Hippospongia, 207,

Hirudmea, 680.

Hirudo, 282, 283, 680.

Histology, 103.

Holoblastic segmentation, 637.
of lancelet, 637.
of starfish, 401.

Hologamy, 596.

Holometabola, 337, 681.

Holophytic, 156.

Holothuroidea, 400, 403.

Holozoic, 136.

786

INDEX

Homologous organs, 691.

Homozygote, 699.

Honey dew, 342.

Hoofs, 574.

Hormones, 21, 62, 64, 673.
male, effect of lack of, 674.

Horns, 577.

structure of, 577.

Horses, 375, 578.

ankle and foot of, 579,
legs of, 579.
skull of, 576.
teeth of, 578.

Host, 179. See also Parasites ;
Parasitism.

House-flies, 343, 346, 347, 348,
as transmitters of disease,
348-

Humour, aqueous, 99.
vitreous, 99.

Hyaloplasm, 104.

Hydatid cyst, 254.

Hydra, 25, 26, 208, 209, 678,
740.

Hydra vindis, H. fusca , H.
grisea, 208.
anatomy of, 208, 210.
development of, 220, 222.
embryology of, 220.
enteron of, 209.
excretion in, 219.
food of, 217, 218.
histology of, 213.
locomotion of, 211.
movements of, 217.
nervous system of, 214, 215,
216.

reproduction of, 219.
tentacles of, 208, 210, 211.

Hydranths, 224.

Hydroids, 223.

Hydrolysis, 10, 12.

Hydrorhiza, 224.

Hydrotheca, 224.

Hydrozoa, 678.

Hylobates entelloides , 585.

Hymenoptera, 342, 681.

Hyoid apparatus, of frog, 44,
45, 629.

arch skeleton of dogfish,
429.

Hyoid apparatus of pigeon,

503.

bone of rabbit, 535.
Hvpapophvsis, 326.

Hypoblast, 607, 639.

primitive, 639.

Hypomere, 613.

Hypopharynx of cockroach,
328.

of mosquito, 343.
of flv, 347.

Hypophysis, 64, 87.
Hvpostome (oral cone), 209,
223, 226.
of tick, 320.

Hyracoidea, 581, 686.
Hyracotherium, 695.

Ichneumon hies, 343.

Identical twins, 675.
Idiochromatin, 596.

Ileum of cockroach, 329.
of frog, 58, 106, 107.
of pigeon, 506.
of rabbit, 544

transverse section of frog’s,

106, 107.

Iliosciatic foramen in the
pigeon, 503.

Ihum, 47, 503, 535.

Imago, 337, 346.

Immunity, 124.

Inbreeding, 602.

Incorporation of food, 8, 17.
Incus, 330.

Individuality, 231-233, 253.
Inflammation, 124, 124.
Infundibulum, 64, 87.
of rabbit, 362.

Inhalant canals of sponges,
203.

Inheritance, 137.

Inhibition, 95.

in earthworm, 264.
in frog, 95.

in nerve net improbable, 21 3.
Ink gland of cuttlefish, 388.
insectivora, 686.

Insects, 317, 322, 334, 681.
classification of, 338, 681.
life-history of, 336.

INDEX

7S7

Insulin, 64.

Intercalary pieces, 426.
Intercellular substance, 113.

Interclavicle, 484.

I ntermediate cell mass, 635,652.
organisms between animals
and plants, 713.

Interrenal body, 459.

Intersexes, 601.

Interstitial cells, 210.

Intervertebral foramen, 39.
notches, 38.

Intestine of Amphioxus, 413.
of crayfish, 300.
of dogfish, 436.
of earthworm, 265.
of frog, 58.
of leech, 284.
of pigeon, 506.
of rabbit, 543, 544.
of swan mussel, 379.

Invertebrates, development of

666.

Iris, 99.

Irritability, 7, 8, 17.
in Amoeba, 144.
in leucocytes, 125.
in plants, 24.
in protoplasm, 102, 126.
of nervous tissue, 95.

Ischiopodite, 289.

Islets of Langerhans, 64.

Isogamy, 154.

Ivory of teeth, 57.

Ixodes, 320, 321, 681.

Jaw, lower. See Mandible
upper. See Skull.

Jaws of frog, 57.

of reptiles, 478, 481.
of vertebrata, 534.

Jelly-fish. See Medusae.

Joints, 50.

“ perfect,” 50.

Juices, gastric and pancreatic,
61, 64.

Kangaroos, 568.

Karyogamy, 16.

Karyokinesis. See Mitosis.

Karyosome, 180, 181.

Katabolism, 12.

Keber’s organs, 381, 381.
Kidney tubules and ducts of
vertebrates, 634.
Kidneys, 19.

development of vertebrate,
548> 635, 636, 652.
of dogfish, 437.
of frog, 78.
of mussel, 380. .

of newt, 475.
of pigeon, 509.
of rabbit, 548.
of reptiles, 484.

Killing, 730.

Kinetonucleus, 183.

Labellae of house-fly, 347.

of mosquito, 344.

Labial palp of cockroach, 324.
Labium of cockroach, 324.

of other insects, 336.

Labrum of cockroach, 322.
of crayfish, 298.
of house-fly, 346.
of mosquito, 346.

Labyrinth —

auditory, of dogfish, 426,
458.

of irog, 42, 92, 100.
of pigeon, 514.
of rabbit, 530, 564.
of tadpole, 626, 627.
carotid, 70.
cartilaginous, 42.
membranous, 42, 100.

Lacerta agilis, 480.

Lacertilia, 484, 685.

Lacinia, 324.

Lactic acid produced from
glycogen, 4, 367.

Lacunae of bone, 122.

Lagena, xoi.

Lamarckism, 703, 705.

Lamella, structureless, 209,
226.

Lamellae of bone, r22.

of gills of mussel, 374.
Lamellibranchiata, 682.

Lamina of vertebra, 38.
terminalis, 89.

788

INDEX

Lamprey, 461, 463.

Lancelet, 404, 405, 683, 748.
alimentary system of, 410.
atrium of, 406.
cleavage of ovum of, 605,
606, 607*

development of, 606-615.
embryology of, 605.
excretory organs of, 414.
external features and habits
of, 404.

gastrulation of, 606, 608.
general anatomy of, 407.
larva of, 61 1.
nervous system of, 416. *
peripharyngeal bands of,
412.

reproductive organs of, 418.
sense organs of, 417.
vascular system of, 415.
Large intestine, 546.

Larva, definition of, 32.
of A carina, 319.
of lancelet, 61 1.

See also Ammoccetes, Bipin-
naria, Brachiolaria,
Glochidium, Life-history
of insects, Planula,
Trochosphere, Tadpole.
Larynx, 75, 506, 629.

Latent bodies of trypano-
somes, 186.

Lateral line of dogfish, 458.

plates, 621, 643.

Latin names, 677.

Law, von Baer’s, 668, 692.
Layers, germ, 610, 625, 639.

of the body, 275.

Lay-out of the body, factors in
the, 669.

Leech, medicinal, 282, 283,

680.

segmentation of, 282.
Leeches, 281, 286. See also
Hirudinea.

Legs, 18, 476.

of Arachnida., 317.
of Araneida, 318.
of cockroach, 324.
of crayfish, 295.
of frog, 34.

Legs of pigeon, 491.
of rabbit, 518.

skeleton of, in vertebrata, 48,
in mammals, 573.
in ungulates, 572, 579.

See also Feet, Limb.

Leishmania, 186.

Lemuroidea, 686.

Lens of frog’s eye, 99.

Lenses. See Microscope.

Lepidoptera, 350, 681.

harm to crops done by, 351.

Lepidosiren, 684.

Leptotene, stage, 133.

Lepus, 517, 677.

Leucilla, 204, 678.

Leucocyte, phagocytic, 123.

Leucocytes, 123.

Lice, 338, 339, 340, 341.

Life, characteristic features of,
16.

continuity of, 726.
definition of, 1.
energy of, 2-7, 721.
origin of, 724, 726.

Life-cycle, 14.
diagram of, 14.

Life-history of, Acalephae, 234,
236.

of A carina, 319.
of Aurelia, 237.
of Entamoeba, 179-181.
of frog, 31, 32.
of house-fly, 348.
of insects, 336.
of lancelet, 61 1.
of liver fluke, 245, 247, 249.
of malaria parasite, 188, 189.
of Monocystis, 158, 159, 160.
of mosquito and gnat, 196.
of Nematoda, 360.
of Nereis, 281.
of Obelia, 224.
of Opalina, 197.
of sea-anemone, 233.
of swan mussel, 384.
of tapeworm, 250, 252.
of Trypanosoma, 184.

See also Reproduction, De-
velopment, Embry-
ology.

INDEX

789

Ligament, ethmopalatine, 429.
falciform, 544.
postspiracular, 429.
suspensory, 436.

Ligaments, 50.

Light, production of, by living
beings, 7.

See also Phosphorescence.
Ligula of cockroach, 324,

Limb, fore, of bird, 504,
of dogfish, 420.
of frog, 34, 533.
of man, 536, 537, 590.
of pigeon, 491, 494.
of rabbit, 536.
of whale, 571.
hind, of bird, 504.
of dogfish, .420.
of frog, 34.
of pigeon, 491.
of rabbit, 518, 538.
pentadactyle. See Penta-
dactyle limbs.

Limbs. See Legs.

of Annelida. See Parapodia.
of arthropoda. See Appen-
dages.

of backboned animals,
422.

of frog, 34.

Limnceus truncatulus, 245.
Linea alba, 51.

Linin, 108, 129.

Linkage, 701.

Lipase, 61.

Liver fluke. See Fluke,
functions of, 62.
of Amphioxus, 413.
of crayfish. See Hepato-
pancreas.
of dogfish, 436.
of frog, 59, 62, 113.
of pigeon, 506.
of rabbit, 544.

Living and lifeless things, I,
720.

beings, definition of, 1, 2.
Lizard, the, 480, 754.
arteries of, 757.
skin of, 481.

skull of, 758.

Lizard, urinary and genital
organs of, 755, 756,
viscera of, 755.

Lizards, 477, 484.

Locomotion of Amoeba, 141.
of A sterias, 392, 397.
of ciliated organisms, 153,
164.

of flagellates, 153.

of Hydra, 211, 217.

of medusae, 227.

of Paramecium, 161.

of Polytoma, 151.

of reptiles, 484, 486.

of snakes, 484.

of the cockroach, 327, 328.

of the crayfish, 297, 298.

of the dogfish, 423.

of the earthworm, 262, 264,

of the frog, 31, 32, 56.

of the leech, 282.

of the pigeon, 495.

of the swan mussel, 369,

373-

See also Walking.

Locomotive organs, 18.

Loop of Henle, 548.

Lophodont teeth, 578.

Love dart of snails, 387.

Lower jaw. See Mandible,
organisms, 20.

Lugworm (Arenicola) , 286.

Lumbricus, 680.

herculeus, 256, 257, See also
Earthworm.

Lung of snail, 385.

-books, 318, 318.

-fishes, 463.

Lungs, 5, 19.
of frog, 75, 76.
of pigeon, 506, 509.
of rabbit, 547.

Lymph, 35, 75.
hearts, 75.
sacs, 35, 36, 75.
vessels, human, diagram of,
589,

Lymphatic vessels of frog, 75,
of rabbits, 557.

Lymphoid tissue, 127.

1 Lysins, 124,

790

INDEX

Macrogametes, 192.
Madagascar, fauna of, 693.
Madreporite, 390.

Malaria parasites, 188.

Male, 15, 600, 601.

gamete. See Gametes.
Malleus, 530.

Malpighian capsule, 78, 633,
636.

layer of skin, 113.
tubules of cockroach, 329.
Maltose, 10.

Mammae, 318.

Mammalia, 460, 366, 686.
classification of, 566-584,
686. 690.

development of, 657.

Man, 585.

arm of, 536, 537.
blood of, 122, 123, 130.
bones of hind limb of, com-
pared with those of
other mammals, 573.
caecum of, 590.
consciousness in, 97.
dentition of, 588.
development of, 661.
morphology of, 588.
skull of, 587, 588, 590.
spinal cord of, 122.

Mandible of cockroach, 323.
of cod. 753.

of crayfish, 293, 298, 302, 303.
of dogfish, 429.
of frog, 44.
of lizard, 756.
of mosquito, 343.
of pigeon, 502.

Mandibular arch. See Arches,
visceral,
fossa, 44.

Mantle of mussel, 371.
Manubrium of Medusa, 224,
226.

stern i, 527.

Manus, 34. See also Hand.
Marginal plates, 486.

Marrow, 122.

Marsupialia, 567, 686.
Mastigophora, 156, 177, 677,
713-

Maturation of gametes, 130.

of ovum, 136.

Maw. See Stomach.

Maxillae of bees, 336.
of cockroach, 324.
of crayfish, 294, 296.
of Diptera, 343, 346.
of Lepidoptera, 335, 350.
of rabbit, 533.

of Vertebrata. See Bone
Skull.

Maxillary palp of cockroach,
324-

of mosquito, 344.

Maxilliped, 294, 295.

Maxillule, 293, 294, 296.

“ Measly ” pork, 252.

Meatus auditorius externus,
530.

Mechanism, as opposed to
vitalism, 723.

Meckel, cartilage of, 44, 429,
533-

Mediastinum, 522.

Medulla oblongata, 87.
of dogfish. 452.
of frog, 87.
of rabbit, 563.
of adrenal bodies, 63.
of green glands, 308.
Medullary plate. See Neural
plate.

sheath, 115.

Medusae, 224, 233.
Meganucleus, 178, 594.
of Balantidium, 196.
of Paramecium, 163, 169,

171- I72-

of V orticella, 175.

Meiosis, 1 31, 133.

Meiotic division, 131.
Membrana semilunaris of
pigeon, 506.

Membrane bones, 40, 44, 528,
629.

Membrane(s) embryonic, 644.
peritrophic, of cockroach, 329.
subzonal, 663.

Mendelism, 695.

Mentum of cockroach, 324.
Merism, 231, 277.

INDEX

79*

Mermis nigrescens, 361.

Meroblastic segmentation. 637.

Merogony, 188.

Meront, 188.

Meropodite, 289.

Merozoite, 188.

Mesenchyme. 234, 625, 642,

653.

Mesenteries of sea-anemones,
233-

Mesenteron (midgut) of As-
ian's, 356, 357.
of chick, 651, 652.
of cockroach. 328.
of crayfish, 300.
of frog embryo, 620.
of tael pole, 628.

Mesentery, 36.

Mesoblast, 607, 620, 643.

Mesoblastic somites (mesoblast
segments), 610, 613, 621,
643, 667.

Mesoderm, 275, 276, See also
Mesoblast.
of earthworm, 275.
of frog, 273.
of liver If like, 243.

Mesoglea, 209, 235.

Mesonephros, 439, 635, 652,

65 3-

of dogfish, 439.
of frog, 633, 636.
of newt, 475.
of rabbit, 348.
of reptiles, 484.

Mesopterygium, 431.

Mesorchium, 80.

Mesothelium. 626.

Mesothorax, 324, 325.

Mesovarium, 80.
of frog, 80.

Metabolic gradient, 233.

Metabolism, 12, 103.

Metabolisms, special, 713.

Metacarpus, 34.

Metachronal rhythm, 153.

Metacromion, 535.

Metagenesis, 230, 248.

Metameric segmentation, 277,
278.

Metamorphosis, 337.

Metamorphosis of insects, 337.
of starfish, 402.
of tadpole, 32, 623.

due to thyroid secretion,
674.

Metanephros, 439, 636, 652,
653-

of pigeon, 509.
of rabbit, 548.
of reptiles, 484.

Metaphase, 134,

Metapleural fold, 404.
Metapophysis, 526.
Metapterygium, 431.
Metastoma, 299.

Metatarsus, 34. See also Legs.
Metatheria, 567, 686.
Metathorax, 324, 323.

Metazoa, 177, 678.

reproduction of, 393.
Microgametes, 192.
Micronucleus, 178, 594.

of Paramecium, 163, 167,

168.

of Vorticella, 173, 176.
Microscope, 730.

use of, 730, 731.

Midgut. See Mesenteron.
Midriff, 521.

Milk, 518, 566.

pigeon’s, 510.

Mill, gastric, 300.

Millipedes, 316, 681.

Mimosa, irritability in, 24.
Miner’s worm, 360.

Miracidium, 243.

Mites, 318, 319.

Mitochondria, 108.

Mitosis, 128, 131.
in Amoeba, 146.
in cells of frog, 12S.
in Paramecium , 167.

Mitotic division, 128, 131.
Mitral valve, 333.

Molar energy, 7.

Molecules, 3.

Molge, 475, 683.

Mollusca, 383, 682.
diagrams of, 385.
evolution of species in, 693g,
695-

792

INDEX

Monocystis, 157, 678, 739.
magna and lumbrici, 157,

158.

cyst of, 159.
dimorphism of, 600.
life-history of, 158, 159, 194.
Monotremata, 566, 686.

development of, 665.
Morphogenesis, 231.
Morphology, 30.

suggests evolution, 689, 690.
Mortality, connection between
cell-specialisation and,
238.

Morula, 659, 659.

Mosaic image, 3 1 1 .

Mosquitoes, 192, 195, 343, 346.
and disease, 190, 362.
compared with gnats, 195.
mouth parts of, 343, 344, 345.
sucking 'blood, 346.

Moths, 35o> 35I-
Mounting, 732.

Mouth, 33. See also Canal,
alimentary.

Movement in animals, 2, 24,
28, 29.

in plants, 24, 28, 29.
Movements in Amceba, 141.

in Paramecium, 165.

Mucous membranes, 1 1 1 .
Mucus, hi.

Mules, 604.

Mullerian duct, 636, 653.
Multiple fission, 147, 175.
Musca domestica, 343, 346.

life-history of, 348.

Muscle, 19, 21, 49.
belly of, 50.
insertion of, 50.
origin of, 50.

-plate, 614.

Muscle or muscles specified —
adductor longus, 54, 33.
rnagnus, 54, 55.
of mussel, 372.
alary, 332.

coccygeo-iliaeus, -52. <

coraco-radialis, 5 1 .
crureus, 55.
deltoideus, 52.

Muscle or muscles specified-
depressor mandibulae, 51.
dorsalis, scapulae, 51.
gastrocnemius, 55.
geniohyoid, 52.
glutaeus maximus, 55.
gracilis major, 55.

minor, 35.
hyoglossus, 32.
iliacus externus, 52, 55.

internus, 54.
latissimus dorsi, 51.
levator scapulae, 51.
longissimus dorsi, 52.
masseter, 52.
mylohyoid, 52.
obliquus; externus, 51.
internus, 51.
superior, of eyeball, 52.
obturator internus, 52, 55.
of Amphioxus, 404.
of crayfish, 297, 300.
of dogfish, 423.
of earthworm, 260.
of frog, 49, 53.
of pigeon, 503.
of swan mussel, 372.
pectineus, 54, 55.
pectoralis, 51.
peronaeus, 55.
petrohyoid, 52.
rectus abdominis, 51.
externus of eyeball, 52.
femoris, 55.

internus of eyeball, 52.
superior of eyeball, 52.
retractor of starfish, 394.
sartorius, 54, 55.
semimembranosus, 55.
semitendinosus, 55.
serratus, 51.
sternohyoid, 52.
temporalis, 52.
tibialis anticus, 55.
transversus abdominis, 51.
triceps brachii, 54.

extensor cruris, 55.
vastus- lateralis, 55.
medialis, 55.- .
muscle fibres. See Muscu
lar tissue.

INDEX

793

Muscular system, details of,
in frog, 49-56-
tissue, 20, 21, 49, 116.
cardiac, 119.
involuntary, 49.
irritability of, 56, 738.
of Ascaris, 354, 356.
of earthworm, 260.
striped, 117, 126.
unstriped, 117, 125.
voluntary, 49.

Musculo-epithelial cells, 210.
Mussel, swan, 369, 370, 371,
372.

anatomy and alimentary
system of, 376, 378.
excretory organs of, 380.
external features of, 372.
feeding of, 372.
gills of. -374.
gonads. of, 381.
habits of, 369.
life-history of, 384.
locomotion of, 372.
mantle of, 371.
nervous system of, 383.
sense organs of, 384.
shell of, 369.
structure of, 380.
vascular system of, 382.
Mussels, freshwater, 369.
Mutation, 702.

Myocoele, 614.

Myocommata, of Amphioxus,

404.

of dogfish, 423.

Myo - epithelial cells. See
Musculo-epithelial.
Myomeres of Amphioxus, 404,

405, 406, 614.
of dogfish, 422, 423.

Myonemes, 157, 174.
Myotome, 615, 629.
Myriapoda, 316, 681.

Names, Latin, 677.

Nares, 518.

external, of dogfish, 419.
of frog, 33.
of pigeon, 491, 501,
external, of rabbit, 518.

Nares, internal of, Choanich-
thyes, 472.
of frog, 57, 102.
of pigeon, 505.
of rabbit, 518, 539, 543.
posterior. See Nares, internal.
Nasal capsules, 39, 41, 42.
duct, 564.

septum in frog, 102.
in rabbit, 539.
Nasopharynx, 540.

Natural selection, 705.
at work, 709.
overriding by, 710.

Nautilus, 388, 682.

Navel, 664.

Neck of rabbit, 518.
Nematocyst, 210, 212, 213.
Nematoda, 352, 680.

and Arthrop'oda, resem-
blances between,' 368.
life-histories of, 360.
Neopallium, 561.

Nephridia, 276, 414

of Amphioxus, 412, 414.
of earthworm, 267, 268, 269,
276.

of leech, 285.

Nephridiopores, 258, 267, 268.
Nephrocoele, 635.
Nephrocoelomostome, 635.
Nephromixium, 276.
Nephrostomes, of earthworm,
267, 268.
of tadpole, 638.

Nephrotome, 635, 652.

Nereis, 279, 680, 717.

cultrifer, 27 8, 279.

Nerve, abducent, 90, 452.
accessory, 563. .
acoustic or auditory, 91, 454.
brachial, 86.
buccal, 454.

chorda tympam, 91, 565.
-components, 93.

-cord of Amphioxus, 416.
of cockroach, 332, 333.
of crayfish, 309.
of earthworm, 260.
of vertebrates. See Spinal
cord.

794

INDEX

Nerve cords of A scans, 355.
of liver fluke, 244,
of starfish, 392.
of Tcenia, 251
depressor, 564.

-endings, 93.
motor, 118.
sensory, 110, 264.
external mandibular, 454.
facial, 90, 563.
fibres, 93, 115.

medullated, 115, 119,

120.

non-medullated, 115.
frontal, 465.

glossopharyngeal, 91, 454,

.565-

hyoidean, 90, 454.
hyomandibular, 90, 454.
hypoglossal, 86, 564.
internal mandibular, 454.
See also chorda tym-
pani.

lateral line, 454, 466.
mandibular, 90, 91, 454,

465-

maxillary, 90, 45a.
nasal, 465.
net, 215, 216.

physiology of, 215.
oculomotor, 90, 452.
olfactory, 90, 452, 563.
ophthalmic, 90, 453, 463,

753-

optic, 90, 452.
palatine, 90, 434.
pathetic, 90, 452.
phrenic, 564.
postspiracular, 434.
post-trematic, 455.
prespiracular, 434.
pretrematic, 453.
recurrent laryngeal, 564.
ring, oral, of starfish, 392.
rings of Medusa, 228.
sciatic, 87.
splanchnic, 563.
superior laryngeal, 564.
trigeminal, 90, 751.'
trochlear. See pathetic,
vagus, 91, 454, 563, 564.

Nerves, 7, 19, 20, 93.

composition of, 93, 115, 453,
.454-

cranial, 41, 84, 90, 92, 452,

455’ 476’ 514. 563-
functions of, 92.
mixed, 92.
motor (efferent), 92.
sensory (afferent), 92.
spinal, 84, 86, 455, 563.
sympathetic, 84, 92, 93, 455,

563-

Nervous impulses, 94, 95.
system, 18.

central, functions of, 95.
formation of, in verte-
brates, 622.
of Amphioxus, 416.
of Ascaris, 355, 356, 357.
of crayfish, 309.
of dogfish, 428, 449, 452.
of earthworms, 260, 262,
264.

of frog, 84, 85.
of Hydra, 214, 215, 216.
of leech, 286.
of liver fluke, 244.
of medusae, 228.
of Nematoda, 357.
of pigeon, 514.
of rabbit, 560.
of skate, 464.
of starfish, 392.
of swan mussel, 3 S3,
of tapeworm, 231.
physiology of, 95.
tissue, 1 15.

Nervures, 325.

Neural crests, 626.
folds, 622, 651.

(medullary) plate, 609
622, 626, 651.
groove, 622.
plates, of turtles, 4S6.
spine, 38.

vessels of earthworm, 270.
Neurilemma, 115.

Neuromast organs, 454, 45S.
Neurone, 115, 116.

Neuronemis, 148.

Neuropodium, 2 So,

INDEX

795

Neuropore, 609.

Newts, 473, 474. 475.

New Zealand, fauna of, 692.
Nictitating membrane, 33.
Nitrogen, circulation of, 714
Nitrogenous waste, 4, 5. See
also Excretion.

Nodes of Ranvier, 115.
Non-cellulat animals, 150, 177.
Nostrils. See Nares, external.
Notochord 418.

of Amphioxus, 408, 609.
of chick, 643.
of dogfish, 424.
of tadpole, 424. 621,
Notopodium, 280.

Notum, 324.

Nourishment of young stages,

665, 666.

Nuchal plate, 486.

Nuclear division, 127 , 597.
membrane, 108.
sap, 108.

Nucleins, 108 in.

Nucleoli, 6, 108.

Nucleoplasm, 103, 109.
Nucleus, 23, 103, 108.
Nutrition, chemosynthetic,
7i4-

holozoic, holophytic, sapro-
phytic, 156.

in plants, 25, 26, 27, 156.
See also Food.

Nyctotherus, 197, 198, 678, 739.
Nymph, 336.
of A carina, 319.
of insects, 336.

Ob eli a, 223, 678, 740.
colony of, 223, 224, 225.
hydranth of, 224, 226.
medusa of, 224, 227, 228,
229,

Obturator foramen, 48, 503.
of pigeon, 503.
of rabbit, 535.

Occipital condyles, 39, 41.

region, 40.

Octopus, 388, 682.

Oculomotor muscles of dogfish,

4 56< 457-

Odonata, 338, 681.

Odontoid process, 324.
(Esophageal pouches, 265.
(Esophagus, 58. See also
Gullet.

of Amphioxus, 413.
of cockroach. 32S.
of crayfish, 299.
of dogfish, 436.
of earthworm, 263.
of frog, 38.
of pigeon, 303.
of rabbit, 343.
of swan mussel, 376.

Offence and defence, organs of,
18.

Offspring, 13, 137, 138, 6 75,
708.

Olecranon, 48, 336.

Olfactory bulb and tract,
560, 561.

lobes, 89, 430, 314, 360.
organs of cockroach, 333.
of crayfish, 314.
of dogfish, 456.
of frog, 102.
of pigeon, 314.
of rabbit, 563.
of swan mussel, 384,
Oligochseta, 680.

Olynthus, 201, 201.

Omentum, hepatic, of dogfish,
436.

Ommatidia, 31 1.

Omosternum, 46.

Onchosphere, 252.

Oocytes, 131.

of Hydra, 219.

Oogamy, 154.

Oogenesis, 130.

Oosperm, 15, 136.

Opalina, 199, 678, 739.

ranarum, 197, 198.

Open circulatory system, 306.
Operculum, of tadpole, 624.

of fishes, 466.

Ophidia, 484, 683.
Ophiuroidea, 400, 4°3-
Opisthonephros, 440, 636.
Opossums, 568.

Opsonins, 124.

796

INDEX

Optic chiasma, 90.
cup, 628.
ganglion, 31 1.
lobes, 87, 452, 514, 562.
thalami. See Thalami.
vesicles, 627.

Oral cone of Hydra, 209.
of Obelia, 223.
hood, 404, 406.

Orbit of dogfish, 426, 456.
of frog, 40.
of pigeon, 500.
of rabbit, 528, 533.
of reptiles, 478.

Ovca gladiator, 569,

Order, 677.

Organ-forming substances in
the ovum, 670.

Organ rudiments,*"' result of
transplantation of, 672.

Organisation, 18, 19, 20, 30,
147, 148.

Organised things, 724.

Organiser, 672.

Organising factors in embry-
ology, 670.

Organism, 19.

Organs, 18.

analogous, 691.
homologous, 691.
serially homologous, 691.
subsidiary, 19.
vestigal, 692.

Origin of life, 724.

of species, 687, 695, 707,
7°9-

Ornithorhynchus , 566, 567.

Oronasal groove, 419, 472.

Orthogenesis, 710.

Osculum, 201.

Orthoptera, 338, 681.

Os innominatum, 535.

Osphradium, 384.

Ossicles, auditory, 529.
brachial, 753.
of crayfish, 300.
of starfish, 389, 394.

Ostia of cockroach, 332.
of crayfish, 302.
of sponges, 204.

Ostracoderms, 460.

Ovaries, 80.

of Ascaris, 357.
of cockroach, 334.
of crayfish, 314.
of earthworm, 270, 273.
of frog, 80.
of Hydra, 219.
of leech, 286.
of liver fluke, 245.
of lizard, 755, 756.
of rabbit, 551.
of whiting, 469.

Ovary of dogfish, 440.
of pigeon, 510.

Oviducts of Ascaris, 357.
of cockroach, 334.
of crayfish, 314.
of dogfish, 440.
of earthworm, 258, 271.
of frog, 82.
of leech, 286.
of liver fluke, 245.
of lizard, 755, 756,
of pigeon, 510.
of rabbit, 551.
of Vertebrata, 469.
of whiting, 469.
origin of, in chick, 653.
in tadpole, 636.

Ovulation, 551.

Ovum, 15.

maturation of, 136.
of bat after entry of sperma-
tozoon, 18,
of birds, 637.
of frog, 1 14.
of mammals, 659.
of man, 13.
segmentation of, 137.

See also Reproduction.

Ox, foot of, bones of, 574,

Oxidation, 3, 5, 9, 10, 367.

Oxygenation of blood, 76.
in frog, 76.

See also Respiration.

Oxyhaemoglobin, 123.

Oxyuris, 365, 365, 680.

Pachytene stage, 133.

Pain, 98.

Palaeozoology, 694.

INDEX

797

Palate, 488, 539.
hard, of man, 584.
of rabbit, 539, 760.

Pallia! line, 371.

Pallium, 89.

Palmar surface, 34.

Palps of Nereis, 280.

labial and maxillary, of cock-
roach, 324.

labial, of mussel, 373, 374.
mandibular, of crayfish,

294-

maxillary, of house-fly,

346-

Palpiger, 324.

Pancreas, 59.
of dogfish, 436.
of frog, 59-
of man, 586,
of pigeon, 505.
of rabbit, 543, 544
Pancreatic duct. See Pan-

creas,
juice, 61.

Papilla, urinary, 438.

urinogenital, 439-
Parabasal body, 184.
Parachordal plates, 629.
Paradidymis, 635 (legend).
Paragaster of sponge, 202.
Paraglossa of cockroach, 324.
Paraglycogen, 157.

Paragnatha, 298, 328.
Paramecium, 678, 739.
aurelia, 163.

caudatum, 161, 162, 163.
conjugation in, 168, 169,

170.

effect of drugs on, 165, 166.
excretion in, 164.
locomotion, 166.
nutrition of, 164.
reproduction in, 167, 594-
595’ 596- 601.
structure of, 161, 162.
Paramylum, 154.

in Euglena, 154.

Parapodia, 280.

of Nereis, 280.

Paraprocts, 325.

Parasites, 365, 366.

Parasites, ciliate, of frog, 197.
hosts of, 366.
life phases of, 366, 367.
protozoan, of man, 179.

See also Acarina, Aphanip-
t e r a, G 1 o c h i d i u m,
Hemiptera, Monocystis,
Nematoda, Platyhel-
minthes.

Parasitism, 365, 366.
Parasympathetic system, 93.
Parazoa, 207, 678.

Parenchyma, 243.
of liver fluke, 243.
of tapeworm, 251.

Parent, 13, 150.

Parents, likeness of offspring
to, 137, 598-

Parietal vessels of earthworm,
270.

lobe, 560.

Paroophoron, 635 (legend).
Parthenogenesis, 16, 248, 593,
594- 596- 597-
artificial, 597.
in Aphis, 340, 341, 593.
in liver fluke, 248, 593.
in Metazoa, 593.
in Protozoa, 594.

Pastern, 578.

Patella, 539-
Pearls, 371.

Pecten in pigeon’s eye, 515.
Pectoral girdle. See Girdle.
Pedicellariae, 389.

Pediculus, 339.
capitus, 339.
vestimenti, 339- 340,
Pedipalpi, 318.

Pelagic fauna, 718, 720.

Pelias berus, 480.

Pellicle of Amoeba, 142, 143.

of Paramecium, 162.
Pelomyxa, 147, 678.

Pelvic girdle. See Girdle.
Pelvis, 535, 536.

Penes of reptiles, 484.

Penial set* of A scans, 353.
Penis of crocodilia, 487.
of leech, 286.
of liver fluke, 245.

798

INDEX

Penis of rabbit, 518, 550, 551.
of snail, 387.

Pentadactyla, cold blooded,

473.

Pentadactyle limbs, 48, 49,
486, 318. 690.
of Amphibia, 474, 684.
of frog, 49.
of Mammalia, 686.
of pigeon, 497.
of rabbit, 518.
of reptiles, 683.

Pepsin, 61.

Peptone, 61.

Per cinema, 136.

Pereiopoda, 293.

Pericardial cavity, coelomic, 63.
blood sinus of cockroach, 332.
of crayfish, 302.
of dogfish, 432.
of frog, 63, 630.
of swan mussel, 376.
of tadpole, 630.

See also Pericardium,
haemoccelic. See Pericardial
blood sinus.

Pericardio-peritoneal canal,
432-

Pericardium, 63.
of frog, 65.
of rabbit, 322, 553.
of snail, 383.

Perihaemal cavity, 399.
rings, 400.

Perilymph, too.

Periostracum, 369.

Periplaneta. See Cockroach,

Perisarc, 224.

Perissodactyla, 375, 578, 686.

Peristalsis, 62.

Peristome of Balantidium, 195.
of Paramecium, 161.
of starfish, 389.
of V or ti cella, 173.

Peristomium of earthworm,
256, 257.
of Nereis, 280.

Peritoneal cavity, 521, 546.
of rabbit, 436, 321.'
funnel, 437, 636.

Peritoneum. 36.

Peritrophic membrane, 329.
Perivisceral cavity, 275.

coelomic, 273. See also
Coelom (perivisceral),
Pericardial cavity (ex-
cept cockroach and cray-
fish), Peritoneal cavity,
Pleural cavities, Pleuro-
peritoneal cavitv.
haemoccelic, 273.

of Arthropoda (=open
blood vascular system),
316, 680.
of Ascaris, 333
of cockroach, 322, 332.
of crayfish, 298.
of Mollusca, 682.
Petromyzon, 463, 683.
Phagocytes, 124.

Pharynx of Amphioxus, 408,
409.

of Ascaris, 336.
of dogfish, 432, 433.
of earthworm, 265.
of frog, 38.
of liver fluke, 241,
of leech, 284.
of Nereis, 280.
of rabbit, 343.

Philosophical questions, 97, 726.
Phosphorescence, 720..
Photosynthesis, 27, 217, 714.
Phthirius inguinalis, 339.
Phyla, definitions of, 677.
Phylum, 677.

Physiological division of
labour, 19.

Physiology, 30.

Pia mater, 84.

Pieces, intercalarv, 426.

pig- 572, 575- 576.

dental formula of, 576.
fore-leg of, 372.

Pigeon, 490, 490, 757..

alimentary system of, 505.
anatomy of, 497, 507. ”
arteries of, 511, 512.
blood vessels of, 510.
excretion and reproduction
of, 509.

external features of, 490.

INDEX

799

Pigeon, feathers of, 492, 493,
494.

musculature of, 503.
nervous system of, 514,
respiratorv organs of, 506,

509.

skeleton of, 497, 498.
skull of, 500, 501, 502.
urino-genital organs of, 508,
509-

wing of, 494, 497.

Pigment cells of eye of cray-
fish, 311.

of retina of frog, 99.
of skin of frog, 35, 64.
spot of Euglena, 155.
of Polytoma, 151.
Pigments, respiratory, 123.
Pinacocytes, 202.

Pineal body or organ, 87.
of frog, 87.
of rabbit, 362.
of Sphenodon, 488.
of tadpole, 87.
stalk in dogfish, 451.

Pinnae of rabbit, 518.

Pisces, 460, 463, 683.

Pituitary body, 64,

function of hormones of, 64,
673-

of chick, 631.
of dogfish, 432.
of frog, 64, 87, 628.
of rabbit, 362.
of tadpole, 628.

Placenta, 352, 663.

Placoid scales, 421, 422.

Plaice, 469, 470.

Planaria, 239, 679.

Plankton, 718.

Plan of study, 30.

Plant cells, 25.

Plantar surface, 34.
Plantigrade, 574.

Plants, 23-30, 151, 152, 156,
714-

differences between animals
and, 24, 29.

Plantula, 325.

Planula, 228.
of Aurelia, 237.

Planula of Obelia, 228,
of sea-anemones, 234.
Plasma of blood, 122.
Plasmagel, 141.

Plasmalemma, 142.

Plasmasol, 142.

Plasmodium, 147.

Plasmodium, 188, 678.
falciparum, 188.

life cycle of, 193.
malarice, 188.
vivax, 188, 191.
life cycle of, 189.
Plasmogamy, 16.

Pla.stogamy, 147.

Plastron of turtle, 485, 487.
Plate, basibranchial, 430.
cribriform, 528.
embryonic, 660.
epicranial, 322.
gill, 622, 623.
lateral, 621, 643.
muscle, 614.

neural or medullary, 609,
622, 626, 651.
segmental, 621
sense, 622.

Plates, parachordal, 629.
podical, 325.
pole, of Amoeba, 146.
of Paramecium, 167.
Platyhelminthes, 239, 679.
Pleura of crayfish, 288.

of cockroach, 324.

Pleurse of rabbit, 521.

Pleural cavities, 521, 322,

546.

Pleurobranchiae, 307.
Pleuronectes, 684,
Pleuroperitoneal cavity, 65,
432, 546, 628.

Plexus, anterior choroid, 88.
brachial, 86.

hepatic, of Amphioxus, 416.
nephridial, of Amphioxus,

4T5-

posterior choroid, 87.
sciatic, 86, 737.
solar, 561, 563, 564.
sympathetic, 93.

Plica semilunaris, 591.

8oo

INDEX

Podical plates, 325.
Podobranchia, 307.

Podomeres, 279.

Polar bodies, 135, 136.

Pole cells, 281, 667.

plates, 146, 167.

Pollen, 24.

Polychaeta, 680.

Polyp, 209.

of Acalephae, 236.
of Obelia, 223.

See also Hydra, Sea-ane-
mones.

Polytoma, 151, 153, 154, 156,
594- 596, 597. 600, 677,
7T3> 739-

encystment of, 152.
food of, 155.
uvella, 15 1, 152.

Pons Varolii, 563.

Pony, fore-leg of, 579.

Pores, abdominal, 420.
dorsal, 259.
of sponges, 201.
spermathecal, 258.

Porifera, 207, 678.

Porocytes, 203.

Portal systems, 73.

hepatic, 73, 416, 446, 448,
512, 513, 632.
renal, 73, 448, 512, 513.
vein. See Vein.
Post-meiotic division, 134.
Postpatagium, 495.

Posture of developing chick,
657-

Postzygapophyses, 38, 524.
Potentialities of blastomeres of
amphibia, 670, 671.
of cells, organ-forming, 670,
671.

Pouch, abdominal, of mar-
supials, 568.

genital, of cockroach, 326.
Practical work, 729.
on Amoeba, 738,

739-

on Ascaris, 745.
on Balantidium, 739.
on Chlamydomonas, 739.
on cockroach, 744,

Practical work on cod, 7 50.
on crayfish, 743.
on dogfish, 749.
on earthworm, 741.
on Entamoeba, 739.
on Euglena, 739.
on frog, 734.
on Hydra, 740.
on lancelet, 748.
on leech, 743.
on liver fluke, 740.
on lizard, 734.
on Monocystis, 739.
on Nyctoherus, 739.
on Obelia, 740.
on Opalina, 739.
on Paramecium, 739.
on pigeon, 757.
on Polytoma, 739.
on Protozoa, 739.
on rabbit, 760.
on snail, 746.
on starfish, 747.
on swan mussel, 746.
on tapeworm, 741.
on V orticella, 739.

Precoxa, 293.

Prementum, 324.

Prepuce, 551.

Preserving fluids, 729.

Presumptive areas, 619, 643.

Prezygapophyses, 38, 524,

526.

Primary body cavity. See
Haemocoele.

Primary remiges, 495.

Primates, 571, 584, 686.

Primitive groove, 620, 623,

642.

streak, 620, 642.

Proamnion, 646.

Proboscidea, 575, 581, 686.

Proboscis of house-fly, 346,

347.

of mosquito, 343.
of various insects, 335.

Proctodaeum, 506, 620, 623,
628. See also Hind-gut

Proglottis, 250, 251, 252.

Prometaphase, 120.

Pronation, 536.

INDEX

801

Pronephros, 439.
of chick, 652.
of dogfish, 439.
of tadpole, 635, 636.
Pronuclei, male and female,
137, 168.

Propatagium, 494.

Prophase, 129.

Propodite, 289.

Propterygium, 431,

Prosoma, 318.

Prosopyles, 204.

Prostate gland of leech, 286.
of rabbit, 551.

Prostomium of earthworm,
256.

of Nereis, 280.

Protease, 61.

Proteins, 9.

composition of, 9.
digestion of, 9, 61.
formation of, by plant, 27.
of the food, 9.
Proterospongia, 200, 201.
Prothorax, 324.

Protoplasm, 18, 22, 103, 126.
fine structure of, 103, 143.
granules of, 106.
properties of, 105.
reactions of, 106.
Protopodite, 289, 292.
Prototheria, 566, 686.
Protozoa, 177, 179, 677.
parasitic in man, 179.
reproduction of, 594.
Proventriculus of crayfish,
299, 299,
of pigeon, 505.

Psalterium, 577.

Pseudobranch, 435, 469.
Pseudo-hearts, 270.
Pseudonavicella, 158, 159.
Pseudopodia, 123, 678.

of Amoeba, 140, 141, 141,
142, 142.

of endoderm of Hydra, 217.
of white corpuscles, 123.
Pseudopodiospores, 159 fn.
Pseudo-tracheae, 347.

Pterylae, 492.

Ptyalin, 543.

51

Pubis, 47.
of frog, 47.
of pigeon, 503.
of rabbit, 535.

Pulex irritans, 349, 349.

Pulp of tooth, 57.

Pulse, 69.

Pupa, 337.

Pupil, 99.

Purposiveness, 17, 720, 723,
726, 727.

Pus, 125.

Putrefaction, 713, 725, 726.

Pygal plates, 486.

Pygostyle, 499.

Pylangium, 66.

Pyloric sphincter, 61.
of dogfish, 436.
of frog, 61.
of rabbit, 543.

Pylorus of frog, 58.
of rabbit, 543.

Pyramids, 563.

Pyriform lobe, 560, 561.

Quill, 493.

Rabbit, 517, 760.

alimentary system of, 539.
anatomy of, 520.
arteries of, 556.
blood of, 557.
blood vessels of, 553.
bones of, 525, 527.
brain of, 558, 559, 560, 560-
circulatory system of, 554.
ductless glands of, 546.
excretory organs of, 548.
external features of, 517.
habits of, 517.
heart of, 552, 553.
intestine of, 543, 544.
lymphatic vessels of, 557
nervous system of, 560.
pelvic girdle of, 536.
placenta of, 552, 663.
races of, 519.

reproductive organs of, 547,
548, 550.

respiratory organs of, 546.
sense organs of, 564.

So2

INDEX

Rabbit, shoulder-girdle of,

529.

skeleton of, 522, 523.
skin of, 520.

skull of, 528, 531, 532,

533.

stomach of, 543.
veins of, 556.
vertebrae of, 522, 525.
Rachis, of feather, 493.

of gonad of A scans, 357.
Radialia, 431, 432.

Radix or pedicle of vertebra,
38.

Radula, 385.

of snail, 388.

Raia, 683.

Rain worm, 361.

Rami communicantes, 737.
of spinal nerves, 86.
of sympathetic system, 92.
Rana temporaria. See Frog.
Rays (fishes), 463.

fin, 430, 431, 432, 474.
gill, 430.

Recapitulation theory, 692.
Receptaculum ovorum, 271.
Receptor cells, 93.

Recessive, 598, 697.
Recombination of genes, 703.
Rectrices, 493.

Rectum. See Canal, alimen-
tary.

Redia, 248.

Reducing division, 131, 136,
168, 199, 273, 699.
Reflex actions, 95.

arc, 96, 96.

Regeneration, 223.
in earthworm, 275.
in Hydra, 223.

Regulation, 21.

of circulation, 21, 22, 70, 92,

564-

of development, 669.
of respiration, 21.
of temperature, 558.
Rejuvenation, 172, 598.
Remiges, 493.

Renal. See Artery, Portal
system, Vein.

Repair of waste of body, 2, 12.

Repeated fission, 152, 175.

Reproduction, 2, 13, 592.
always involves fission, 13.
analysis of, 13.
asexual, 16, 592, 593, 595.
of Amoeba, 146, 594.
of Amphioxus, 418.
of Ascaris, 357.
of Carchesium, 177.
of Chlamydomonas, 133.
of cockroach, 334.
of crayfish, 314.
of Entamoeba, 179.
of frog, 80, 136.
of Hydra, 219.
of leech, 286.
of liver fluke, 244.
of malaria parasite, 188.
of Metazoa, 593.
of Monocystis, 158.
of Nereis, 280.
of Olynthus, 203.
of Opalina, 197.
of Paramecium, 167, 594,

595. 596, 601.
of pigeon, 509.
of Polytoma, 152, 594.
of Protozoa, 594.
of rabbit, 548.
of starfish, 401.
of swan mussel, 384.
of tapeworms, 252.
of Trypanosoma, 186.
of Vorticella, 175.
sexual, 16, 592, 593. See
also Life-history,
sexual and asexual con-
trasted, 592, 593.
of plants, 24.
problem of, 595.

Reproductive bodies, 13.
organs. See Generative
organs.

Reptilia, 460, 475, 685.
blood vessels of, 478, 484.
heart of, 481.
orders of, 484.
skull of, 477, 482, 483.
urinogenital organs of, 479,
484.

INDEX

803

Resemblance of offspring to
parents, 13. See also
Heredity.

Respiration, 5, 16 fn., 19.
different meanings of, 5 fn.
of Amoeba, 145.
of Amphibia, 484.
of birds, 484.
of cockroach, 330.
of crayfish, 306.
of dogfish, 437.
of earthworm, 269.
of frog, 75, 76.
of Hydra, 219.
of mammals, 484.
of pigeon, 506.
of plants, 27.
of rabbit, 546.
of reptiles, 476, 484.
of turtles, 484.

Respiratory pigments, 123.

Response. See Stimuli.

Restiform bodies of dogfish,
452.

Reticulum, 577.

Retina, 99.
origin of, 628.

Retinal image, 99, 100.

Retinula, 31 1.

Rhabditis, 360.

R.habdonema nigrovenosum,
365-

Rhinencephalon, 560.

Rhizopoda. See Sarcodina.

Rhynchocephalia, 488, 685.

Ribs, of cod, 751.
of dogfish, 426.
of pigeon, 497, 499.
of rabbit, 526.
of snakes, 484.
of turtles, 484, 486.
vestigial, of frog, 38.

Rima glottidis, 75.

Rodentia, 583, 686.

Rods and cones, 110, in.

Rostrum of Amphioxus, 404.
of crayfish, 288.
of dogfish, 426.
of Ixodes, 320.

Rumen, 576.

Ruminantia, 576, 686.

Sacculus of frog’s ear, 100.
rotundus, 544, 546.

Saccus vasculosus, 452.

Sacs, air, of pigeon, 508.
lymph, of frog, 35.
scrotal, of rabbit, 518, 550.
sperm, of dogfish, 439.
vocal, of frog, 58.

Salamandra, 475, 685.

Saliva, 6.

Salivary glands. See Glands,
salivary.

Salmo, 684.

Salmon, 468.

Salts, inorganic, of the food,
9-

Saprophytic organisms, 156.

Sarcodina, 177, 678.

Sarcolemma, 118.

Savcoptes, 319, 320, 681.

Sawflies, 338, 343.

Scales, cycloid, 468.

epidermal, of pigeon, 491.
of reptiles, 475.
of snakes, 484.
of turtles, 486.
ganoid, 468.
of dogfish, 422.
of Gymnophiona, 474.
placoid, 421, 422, 463.

Scaphognathite, 294, 307.

Scapus, 493.

Schistosoma, 249, 679.
hcematobium, 249.

Schizogony, 188.

Schizont, 188.

Schizozoite, 188.

Sclerites, 297.

Sclerotic, 98.

Sclerotome, 615, 625, 629,

643*

Scolex, 250.

Scorpionida, 681.

Scy Ilium, 419, 683.

Scyphozoa, 679.

Scytomonas. See Copvomonas.

Sea cows, 571.
cucumbers, 403.
lilies, 403, 682.
urchins, 403, 682.

Sea-anemones, 233, 679, 717.

804

INDEX

Secondary or true body cavity.
See Coelom,
remiges, 495.
sex characters, 601.

effect of ovarian and tes-
ticular hormones on,
674.

Secretins, 64.

Secretion, 7, 171, 721.
in plants, 24.
internal, 22, 62.

Section cutting, 732.

Segmental plates, 621.

Segmentation, 277.

metameric, 277, 278. See
also Somites,
of Amphioxus, 404.
of Arthropoda, 316, 680.
of dogfish, 421, 422.
of earthworm, 277, 278.
of frog, 521.

of ovum. See Cleavage,
of rabbit, 521.
of tapeworm, 278.
of Vertebrata, 422.

Segments of body. See
Somites.

of limbs. See Podomeres.
of ovum. See Blastomeres.

Segregation, 697.

Selenodont teeth, 576.

Self-differentiating rudiments,
672.

Self-fertilisation, 158, 599.

Sella turcica, 528.

Semicircular canals, 100, 101,
426.

Semilunar valves of heart of
dogfish, 444.
of frog, 66.
of pigeon, 512.
of rabbit, 554.

Seminal- vesicle of man, 588.

Seminiferous tubules, 80, 113.

Sense organs, 18.

of Amphioxus, 417.
of cockroach, 333.
of crayfish, 31 1.
of dogfish, 428, 456.
of earthworm, 265.
of frog, 98.

Sense organs of pigeon, 514.
of rabbit, 564.
of swan mussel, 384.

See also Ear, Eye, Cnidocil,
Olfactory organs, Stato-
cyst.

plate, 622.

-tentacle of starfish, 392.
Senses of Amoeba, 144.
of crayfish, 31 1,
of dogfish, 456.
of frog, 98.
of man, 585.
of rabbit, 564.
of Vertebrata, 98.

See also Sense organs.
Sensibility, general, 98.

Sepia, 388, 682.

Septa of earthworm, 260, 261.
Septum, interorbital, of pigeon,
500.

of reptiles, 478.
nasal, 102, 426, 532, 539.
Sero-amniotic connection, 649.
Serum, 127.

Sessile animals, 172.

Setae, coxopoditic, of crayfish,
289.

of earthworm and Nereis.
See Chaetae.

Setobranch, 289.

Sex, 14, 599, 674.

a Mendelian character, 701.
determination of, 601.
Sexual characters, secondary,
601, 674.

differences, secondary, in
insects, 603.

reproduction. See Repro-
duction, sexual.

Sheep, 576.
heart of, 761.
skull of, 575.
stomach of, 575.

Shell, egg. See Reproduction,
gland of dogfish, 440.
of liver fluke, 245.
of swan mussel, 369.
of various molluscs, 385, 388.
Shield of crayfish. See Cara-
pace.

INDEX

805

Sinu-auricular valves of frog’s
heart, 65.

Sinus, pericardial, of crayfish,
302.

sternal, of crayfish, 305.
terminalis, 653.
urinary, of dogfish, 438.
urinogenital, of dogfish, 439.
venosus, of chick embryo,
654-

of dogfish, 442, 446.
of frog, 65, 68, 72.
of reptiles, 481.
of tadpole, 631.

Sinuses, vascular, of cock-
roach, 332.
of crayfish, 305.
of dogfish, 446.
of leech, 285.

Siphonoglyphs, 234.

Siphons of mussel, 371.

Sirenia, 571, 686.

Skate, 462, 463.

blood vessels of, 465.
heart of, 463.
nervous system of, 464.
various organs of, 463.

Skates, 463.

Skein (or spireme), 129.

Skeletal organs, 19.

Skeleton, 19, 49.

appendicular, 36, 522.
axial, 36, 424, 522.
endophragmal, 297.
of cod, 753.
of crayfish, 297.
of dogfish, 424, 427.
of frog, 36, 37.
of pigeon, 497, 498.
of rabbit, 522, 523.
of skate, 462.
of tadpole, 629.
preparation of, 733.
visceral, of dogfish, 429.

Skin of bird, 493.
of cod, 750, 751.
of dogfish, 422.
of frog, 34, 75, 114.
of lizard, 481.
of mammal, 520.

“ scarf,” 1 13.

Skull, 421.

Skull of Amphibia, 475.
of bird, 500.
of Capitosaurus, 477.
of dog, 528, 580, 581.
of dogfish, 425, 426.
of frog, 36, 39, 39, 40, 41, 42,
43.

of horse, 576.
of lizard, 756, 758.
of mammal, 530.
of man, 587.
of pigeon, 500, 501, 502.
of rabbit, 528, 531, 532,
533.

of reptiles, 477, 482, 483.
of sheep, 575.
of tadpole, 629.
of turtle, 488.
of whales, 570.
of whiting, 753.

Sleeping sickness, 184, 185,

1 86.

Slits, gill-, of Amphioxus, 406.
of dogfish, 419.

Sloths, 571.

Snail, the, 385, 386.
alimentation of, 388.
reproductive system of, 387.
sexual congress in, 602.

Snails, 385.

Snake, skeleton of a, 485.

Snakes, 477, 484.

Soil, action of earthworms on,
256, 721.

Solar plexus, 561, 563, 564-

Solenocytes, 270, 414.

Soles (fishes), 469.
of feet, 34, 518.

Sols, 105.

Somatopleure, 613, 647.

Somites, 256, 277, 279.

mesoblastic. See Meso-
blastic somites,
true, 275.

of cockroach, 322.
of crayfish, 287.
of earthworm, 256, 257,
277-

Somites, true, of leech, 282.
of Nereis, 279.

8o6

INDEX

Somites. See also Segmenta-
tion, metameric-
Special creation, 687, 724.
Specialisation, 20.

and mortality, connection
between, 238.

Species, 676.

Sperm, 16.
rosettes, 273.
sacs of dogfish, 439.
Spermatheca, 271.
Spermathecal pores of earth-
worm, 258.

Spermatic cord, 550.
Spermatids, 134.
Spermatocytes, 131.
Spermatogenesis, 130, 134.
of earthworm, 271, 272,

273-

of rat, 762.

Spermatophore of snail, 387.
Spermatozoon, 15.
of Ascaris, 357.
of crayfish, 315, 316.
of earthworm, 271, 272,

273-

of frog, 80, 1 13, 115, 130.
of Hydra, 220.
of man, 16.
of Nematoda, 368.
Sperm-mother cells of earth-
worm, 271.

Sperm rosettes, 273.

Sphenodon, 488.

Sphenoidal region, 40.
Sphincter, pyloric, of dogfish,
436.

of frog, 61.
of rabbit, 543.

Spiders, 318, 318, 681.

Spinal cord, 421.

diagram of fibres entering
and leaving, 117.
of dogfish, 449.
of frog, 36, 84, 88.
of man, 122.
of rabbit, 563.
nerves, 84, 86, 88, 455, 563.
Spindles, cell-, 129.

Spine. See Backbone,
haemal, of dogfish, 426.

Spine, neural, 38, 524, 525, 526.

of scapula of rabbit, 535.
Spines of starfish, 389.
Spinnerets, 318.

Spinules of liver fluke, 241.
Spiracle, 419, 435, 436, 463,
466, 628.

Spiral valve of conus arterio-
sus, 66.

of intestine, 436, 437, 466.
Spireme, 129.

Splanchnoccele, 614.
Splanchnomere, 613.
Splanchnopleure, 613, 646, 647.
Spleen, 64.

of dogfish, 437.
of frog, 64.
of pigeon, 506.
of rabbit, 546.

Sponge, bath, diagram of
structure of, 206.

Sponge bodies, complex, 203.
simple, 201.

structure, grades of, 205.
Sponges, 200-207.

branched calcareous, 203.
canal systems of, 203, 204,
206.

dermal layer of, 202.
food of, 203.
gastral layer of, 202.
larvae of, 207.
osculum of, 201.
pores of, 201.
siliceous and horny, 205.
skeletogenous layer of, 202.
spicules of, 202.
Spongioplasm, 104.

Spore formation in Amoeba,
147-

in Monocystis, 1 59.

Spores, 147, 159 fn., 201, 726.
Sporoblast, 192.

Sporocyst, 246.

Sporont, 158, 192.

Sporozoa, 177, 678.

Sporozoites of Monocystis, 159.

of Plasmodium , 192.

“ Staggers,” 254.

Staining, 731.

Stapes, 530.

INDEX

807

Starch, 10, 27, 28, 151, 217,
7I3- .

granules in Polytoma, 151.
Starfish, 389, 747.

alimentary canal of, 394.
body-wall of, 392.
coelom of, 392.
diagrams of sections of, 393,
398.

external features of, 389.
food of, 394.
larva of, 400, 402.
nervous system of, 392.
perihaemal and pseudohaemal
systems of, 400.
reproduction of, 401.
sense-tentacles of, 392.
water vascular system of,
396, 396.

Statocysts of crayfish, 292, 31 1,

313.

of medusa, 228.
of mussel, 383.

Stearic acid, 10.

Stearin, 10.

Stegocephali, 474, 475, 684,
Stenohaline animals, 396.
Sterilisation, 725.

Sterna of crayfish, 288.
Sternum, absent in dogfish,
424.

of lizard, 759.
of Ornithorhynchus , 567.
of pigeon, 502.
of rabbit, 526, 529.
structures representing, in
frog, 46.

Stigma (eyespot, pigment spot)
of flagellates, 28, 151,
155-

Stigmata of cockroach, 330.
Stimuli, 8, 17, 94, 95, 96.

721.

effects of, on Amoeba, 144.
on cnidocils, 212.
on Hydra, 215, 218.
on muscle, 56, 738.
on nerve, 94, 738.
on nerve net, 215.
on Paramecium, 165.
Stipes, 324.

Stomach, 18.
of crayfish, 299.
of dogfish, 436.
of frog, 58.
of leech, 284.
of medusae, 226, 233.
of mussel, 376.
of rabbit, 543.
of ruminants, 576, 577,
of sheep, 575.
of starfish, 394.

Stomodaeum, 623, 628, 651.
See also Fore-gut.

Strepsitene condition, 120.

Strobilae, 279.

Strobilation, 236, 278.

Structure and function, 18.

Struggle for existence, 17,
707, 720.

Sturgeon, 466.

Style, crystalline, 377.

Styles of cockroach, 326.

Subclass, 677.

Subgenital pits, 236.

Subkingdom, 677.

Submentum, 324.

Subphylum, 677.

Subplantigrade, 574.

Subumbrella, 224.

Subzonal membrane, 663.

Succus entericus, 61.

Suckers of leech, 282.
of liver fluke, 241.
of tube feet, 392.

Sugar, 10, 21, 22, 27, 62, 546.
cane, 10.

grape. See Glucose,
malt, 10.

Suina, 576, 686.

Sulci, cerebral, 560.

Superlinguae, 328.

Supination, 536.

Suprarenal gland, 63, 459. See
also Adrenal bodies.

Surface action, 105.

Suspensorium, 43.

Sutures of skull, 528.

Swan mussel. See Mussel,
swan.

Swimming. See Locomotion.

Sycon, 203, 678.

8o8

INDEX

Sylvian fissure, 560.

Symbiosis, 217, 546.
Symmetry, 33, 403.
bilateral, 33, 403.
radial, 226, 403.
Sympathetic system, 84, 92,
93-

of dogfish, 455.
of frog, 84, 92, 93.
of rabbit, 563, 564.
Symplasts, 147 fn.

Synangium, 66.

Synapse, 94, 117.

Synapticulae, 412.

Syncytium, 147, 296.
in Ascaris, 354.
in egg of crayfish, 315.
in epidermis of crayfish,
296.

Syngamy, 14, 15, 16, 592-599.
See also Conjugation,
problem of, 596-599.
varieties of, 154.

Synovia, 50.

Synovial capsule, 50.

Syrinx, 506.

Tadpole, 31, 623.

Tcsnia, 679.

life-history of, 250.
ccenurus, 253.
echinococcus, 254.
saginata, 253.
serrata, 253.
solium, 249.

Tail, 405, 418.

of Amphioxus, 405.
of Cetacea, 571.
of dogfish, 419, 420.
of fishes, 466.
of newts, 474.
of pigeon, 490.
of rabbit, 517.

of tadpole, 31, 418, 623, 624,
625.

vestigial vertebrae of. See
Coccyx, Pygostyle, Uro-
style.

Tail-fan of crayfish, 295.
Tapeworms, 249, 741.

Tapirs, 578.

Tarsus of cockroach, 323, 324.
of vertebrates, 34. See also
Skeleton of frog, pigeon,
rabbit.

Taste-buds, 565.

Taxis, 166.

Tears, 6, 564.

Teeth, 18, 422.
bunodont, 574.
canine, 541.
cheek, 541.
incisor, 541.
lophodont, 578.
milk, 541.
molar, 541.
of dogfish, 422, 432.
of elephant, 581.
of frog, 57.

of horse, structure of, 578.
of mammals, 540, 541, 574,
583-

of man, upper, 584.
of pig, 541.
of rabbit, 540, 541.
of whales, 571.
permanent, 541.
premolar, 541.
selenodont, 576.

Tegenaria, 681.

Teleostei, 468, 684.

Teleostomi. See Actino-
pterygii, Choanichthyes.
Telophase, 129.

Telson, 288.

Temporal lobe, 560.

Tendo Achillis, 55.

Tendon tissue, 120, 121.
Tendons, 50.

Tentacles of Hydra, 208.
of medusa, 224, 226, 227.
of Obelia polyp, 223.
of sea-anemone, 234.
prostomial, of Nereis, 280.
sense, of starfish, 392.
velar, of Amphioxus, 406.
Tergum of cockroach, 324.

of crayfish, 288.

Testes, 80.

function of hormone of, 674.
of Ascaris, 357.
of cockroach, 333.

INDEX

809

Testes of crayfish, 314.
of dogfish, 440.
of earthworm, 271.
of frog, 80.
of Hydra, 219.
of leech, 286.
of liver fluke, 244.
of lizard, 755, 756,
of pigeon, 509.
of rabbit, 550.
of reptiles, 484.
of Teleostei, 484.

Tetrapoda, 474.
Thalamencephalon of dogfish,
45i-

of frog, 87, 90.
of pigeon, 514.
of rabbit, 562.

Thalami, 87, 562.

Thoracic duct of rabbit, 557.
Thorax of cockroach, 322, 324.
of crayfish, 287.
of rabbit, 521, 546.
Thrombin, 127.

Thymus, 64, 458.
of dogfish, 458.
of frog, 64.

of rabbit, 522, 546, 553.
Thyroid gland, 63, 458, 463,
673-

effect of secretion of, 62, 63,
673. 674.
of dogfish, 444.
of frog, 63.
of rabbit, 546.

Thyroxin, 63, 673.

Ticks, 318, 320.

Tiger moth, head of, 349,
Tissue, 20, 103.
adenoid, 127.

adipose or fatty, 120, 129.
connective, 20, 120, 121, 128.
epithelial, 109.
lymphoid, 127.
mesoblastic, 628.
muscular. See Muscular
tissue.

nervous, 115.

portions of, showing cells, 23.
Tissue, skeletal, 119.

various forms of, 20, 109.

Titanotheres, 710.

Tongue of frog, 57.
of pigeon, 505.
of rabbit, 540.

Tonsil of rabbit, 540.

Tortoises, 485.

Trabeculae of skull, 629.

Trachea, 75.
of frog, 75.
of pigeon, 506.
of rabbit, 546.

Tracheae of Acarina, 319.
of cockroach, 330.

Tracheoles of cockroach, 330.

Transverse processes of verte-
brae, 38. See also
Vertebrae.

Trematoda, 240, 241, 249,

679.

“ Trial and Error,” method of,
166.

Trichinella, 363, 364, 366, 680.

Trichocephalus, 364, 365.

Trichocysts, 162.

Trichuris. See Trichocephalus.

Tricuspid valve, 553.

Triploblastica, 275, 625, 679,
682.

Trcchlea, 47.

Trochosphere, 281, 281.

Trophoblast, 660.

Trophochromatin, 596.

Trophonucleus, 183.

Trophozoite, 158, 188, 194.

Truncus arteriosus, 65.

Trunk of frog, 33.

Trypanosoma, 183, 677.
brucei, 186.
cruzi, 187.
equinum, 187.
gambiense, 184, 184, 185.
rhodesiense, 187.

Trypsin, 61.

Tryptophane, 9.

Tsetse flies, 343.

Tuatara, 477, 478, 481, 488,
489.

Tube feet, 403.
of starfish, 392.

Tuberculum, or tubercle, of
rib, 499, 526.

8io

INDEX

Turbellaria, 239, 679.

Turtles, 477, 484, 486.
Tylenchus, 680.

scandens, 360, 361, 366.
Tympanic cavity, 101, 530,

628.

membrane, 101.

Typhlosole of earthworm, 265.
of swan mussel, 380.

Umbilical stalk, 649.
Umbilicus, superior and in-
ferior, of feather, 493.

494-
Umbo, 370.

Undulating membrane of Para-
mecium, 161.
of Trypanosoma, 183.
Unguiculata, 572, 583.
Ungulata, 571, 574, 686.
Unguligrade, 574.

“ Unicellular ” organisms, 149,
I77-

Unorganised things, 724.

Urea, 4, 80, 164, 448, 449.
Ureter, 636.
of lizard, 755.
of pigeon, 509.
of rabbit, 548.

(so-called) of dogfish, 438.
of frog, 79 n.

Urethra, 551.

Urinary bladder. See Bladder,
organs. See Kidneys,
papilla, 438.
sinus, 438.

Urine, 5, 7, 79.

Uriniferous tubules, 78, 78.
Urinogenital papilla, 439.
sinus, 439.

system of vertebrates. See
Kidneys. See also 484,

547,

Urodaeum, 506.

Urodela, 474, 685.

Uromastix, 756.

skull of, 758.

Urostyle, 36.

Uterus, human, 588.
masculinus, 551.
of Ascaris, 357.

Uterus of liver fluke, 245.

of rabbit, 551.

Utriculus of frog’s ear, 100.

Vacuoles, 106.

contractile of Amoeba, 140,
145-

of Balantidium, 196.
of Euglena, 155.
of Paramecium, 164, 164.
of Polytoma, 15 1.
of Vorticella, 175.
food, of Amoeba, 144.
of Paramecium, 165.
of Vorticella, 175.
formative, 164.

Vagina of Ascaris, 357.
of leech, 286.
of marsupials, 568.
of rabbit, 551.

Valve, Eustachian, 654.
Valves, heart, of crayfish, 302.
of dogfish, 444.
of frog, 65.
of pigeon, 512.
of rabbit, 553.
of veins, 69.

semilunar. See Semilunar

valves.

shell, of swan mussel, 369.
spiral, of dogfish gut, 436.

of conus arteriosus, 66.
watch-pocket. See Watch-
pocket valves.

Vane (or vexillum) of feather,
494-

Variation, 675, 687, 707.
acquired, 707.
germinal, 707.

Vas deferens of man, 588.

V asa deferentia of crayfish, 315.
of chick, 653.
of dogfish, 440.
of earthworm, 258, 273.
of frog, 80.
of pigeon, 510.
of rabbit, 551.
of reptiles, 484.
of teleostei, 484.
of vertebrates, 484.

See also Wolffian duct.

INDEX

Si i

Vasa efferentia of frog, 80, 636.

of dogfish, 440.

Vascular system, 18.

of chick embryo, 653, 654,

655.

of cockroach, 331.
of crayfish, 302.
of earthworm, 269.
of tadpole, 629.

See also Arteries, Veins.
Vegetative pole, 605.

Vein —

allantoic, 654, 663.
anterior abdominal, 73, 514.
azygos, 556.
brachial, 72, 512.
caudal, 448, 512.
coccygeo-mesenteric, 512.
dorsolumbar, 73.
epigastric, 512.
external iliac, 556.

jugular, 72, 556.
femoral, 73, 512.
genital, 556.
hepatic, 73, 556.

portal. See Portal,
hypogastric, 512.
iliolumbar, 556.
innominate, of frog, 72.
of man and sheep, 591,
761.

internal iliac, 512, 556.

jugular, 72, 556.
jugular, 512.
lingual, 72.
mandibular, 72.
musculocutaneous, 72.
ovarian, 73.
pectoral, 512.
pelvic, 73.

portal, 73, 512, 556.
postcaval (or inferior vena
cava), 72, 474, 512, 556,
654-

precaval (or superior vena
cava), 72, 512, 556.
pulmonary, 71, 553.
renal, 72.

portal, 73, 512, 556.
sciatic, 73, 512.
spermatic, 73.

V ein — -

subclavian, 72.
subscapular, 72.
vesical, 73.

vitelline, 630, 653, 654.
Veins, 69.

of dogfish, 446, 447, 448,
555.

of embryo chick, 630, 653,
654, 655.

of frog, 71, 72, 73.
of pigeon, 512.
of rabbit, 555, 556.
of reptiles, 484.
of tadpole, 630.
of vertebrates, 555.

See also Sinuses.

Velum of Amphioxus, 406.

of medusa, 224.

Vena cava of swan mussel,
382.

inferior, of chick embryo,
654-

Venae cavae of Vertebrata. See
Vein, postcaval, pre-
caval.

Venous blood, 73, 76.
system. See Veins.

Vent or cloacal opening, of dog-
fish, 405, 436.
of frog, 34.
of pigeon, 492.

Ventral surface, 33.

Ventricle of heart, 65.

Ventricles of brain, 87, 89.

Ventrilateral processes, 424,
426.

Venules, 69.

Vermiform appendix, 546, 590.

Vermifuges, 358.

Vermis, 563.

Vertebrae, 36.
of dogfish, 424.
of frog, 36, 38, 38.
of pigeon, 499.
of rabbit, 522, 524, 525, 526,
of snakes, 485.

See also Backbone.

Vertebral canal, 36, 38.
column. See Backbone,
foramen, 38.

812

INDEX

Vertebr arterial canal, 524.

Vertebrata, 418, 421, 683.
cold-blooded, 460.

Vesicle, blastodermic, 660.
cerebral, of Amphioxus, 417.

Vesiculae seminales, 271, 273.
of dogfish, 438.
of earthworm, 271, 273.
of frog, 80.

Vestibule of auditory laby-
rinth, 100.
of Amphioxus, 406.
of Paramecium, 161.
of rabbit, 551.

of various Ciliata. See
Gullet.

of Vorticella, 173, 175.

Vestigial organs, 692.

Vexillum (or vane) of feather,
494.

Vibrissae, 518.

Villi of rabbit’s intestine, 544.
of trophoblast, 661, 663.

Vinegar worm, 360.

Viscera, 35.

Visceral arches, 436.

of tadpole, 39, 628.
clefts, 418, 651, 668.

of the chick embryo, 654.
See also Gill clefts,
Spiracle,
hump, 376, 385.
nerves of cockroach, 332.
of crayfish, 310.
of dogfish, 454, 455.
of frog, 84, 92.
of vertebrata, 93.

See also Sympathetic ner-
vous system.

skeleton of dogfish, 425, 429.
of rabbit, 533, 535.
of tadpole, 629.

Vision in crayfish, 31 1.
in frog, 99.

Visual purple, 100.

Vitalism, 723.

Vitality, suspended, 145, 724.

Vitamins, 10, n.

curve showing effect of on
growth, 11.
role of, 11.

Vitelline membrane of ovum,
of Amphioxus, 605.
of bird, 638.
of frog, 1 14, 136.

Vitreous humour, 99.
Viviparous animals, 553.
aphides, 340.

nematodes, various, 361,
362, 363.

Vocal cords, 75.
sacs, 58.

Voluntary action, 95, 96.

Von Baer’s law, 668, 692.
Vorticella, 172, 173, 174, 601,
678- 739-

imitation of sex in, 176, 601.
Vulva, 518, 551.

Walking, 476. See also Loco-
motion.

Warm-blooded animals, 74,

5IO> 558- 566.

Wasps, 337, 343.

Waste products of body, 6.

See also Excretion.
Watch-pocket valves, 66, 69.
See also Semilunar
valves.

Water as food, 9.
as medium, 71 1.
in animal body, 5, 78, 79,
7H, 712-

in protoplasm, 104, 720.
in Protozoa, 145.
snail, 245.

Wear and tear, 7.

Webs of frog’s foot, 34.

Whale, the killer, 569.

the sperm, 571.

Whalebone, 570, 571.

Whales, 568, 571.

skulls of, 570.

Wheel organ, 406.

Whelks, 385.

White of egg, 510, 638, 657,

666.

matter, 84.

of brain, 89, 116.
of spinal cord, 84.
Whiting, 468, 750.

Windpipe. See Trachea.

INDEX

813

«

Wings, 18.

as homologous and analo-
gous organs, 691.
of bat and bird compared,

582.

of cockroach, 322, 325, 327.
of insects, 336.
of pigeon, 494.

Wolffian duct, 79, 80, 437, 439,
440, 479, 484, 510, 548,
636, 652, 653.

Wombats, 568.

Xiphisternum, 46, 527.
Xiphoid cartilage, 46, 527.

process, 527 .

Xylol, use of, 732.

Yeast, 3, 713.

Yellow cells, 266.

Yolk, 114, 510, 605, 616, 637,
659, 665.

Yolk cells, 245.
duct, 245.
glands, 245.
plug, 619.

sac, 647, 657, 659, 661, 664.

Zebras, 578.

Zona radiata, 659.

Zoochlorella , 217.

Zooids, 177, 231.

Zoology, 1, 30.

Zygantra, 485.

Zygapophyses, 38, 524.
Zygoma, 533.

Zygomatic arch, 529, 533.
process, of maxilla, 529, 533.
of squamosal, 529, 533.
Zygosphenes, 485.

Zygote, 15, 134, 136, 596, 601,
699, 700, 701.

Zygotene stage, 133.

Zymogen, 112.

s